diff --git a/.gitignore b/.gitignore
index 9050bdab9..346ef9afa 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,3 +3,4 @@ target/
 Cargo.lock
 *.pk
 *.sk
+*.raw_keypairs
diff --git a/beacon_node/beacon_chain/src/beacon_chain.rs b/beacon_node/beacon_chain/src/beacon_chain.rs
index 3d2efa8ae..b0e84e1e1 100644
--- a/beacon_node/beacon_chain/src/beacon_chain.rs
+++ b/beacon_node/beacon_chain/src/beacon_chain.rs
@@ -73,31 +73,18 @@ where
     F: ForkChoice,
 {
     /// Instantiate a new Beacon Chain, from genesis.
-    #[allow(clippy::too_many_arguments)] // Will be re-factored in the coming weeks.
-    pub fn genesis(
+    pub fn from_genesis(
         state_store: Arc<BeaconStateStore<T>>,
         block_store: Arc<BeaconBlockStore<T>>,
         slot_clock: U,
-        genesis_time: u64,
-        latest_eth1_data: Eth1Data,
-        initial_validator_deposits: Vec<Deposit>,
+        mut genesis_state: BeaconState,
+        genesis_block: BeaconBlock,
         spec: ChainSpec,
         fork_choice: F,
     ) -> Result<Self, Error> {
-        if initial_validator_deposits.is_empty() {
-            return Err(Error::InsufficientValidators);
-        }
-
-        let mut genesis_state = BeaconState::genesis(
-            genesis_time,
-            initial_validator_deposits,
-            latest_eth1_data,
-            &spec,
-        )?;
         let state_root = genesis_state.canonical_root();
         state_store.put(&state_root, &ssz_encode(&genesis_state)[..])?;
 
-        let genesis_block = BeaconBlock::genesis(state_root, &spec);
         let block_root = genesis_block.canonical_root();
         block_store.put(&block_root, &ssz_encode(&genesis_block)[..])?;
 
diff --git a/beacon_node/beacon_chain/test_harness/Cargo.toml b/beacon_node/beacon_chain/test_harness/Cargo.toml
index bd7a58270..448934eb3 100644
--- a/beacon_node/beacon_chain/test_harness/Cargo.toml
+++ b/beacon_node/beacon_chain/test_harness/Cargo.toml
@@ -33,12 +33,14 @@ failure = "0.1"
 failure_derive = "0.1"
 fork_choice = { path = "../../../eth2/fork_choice" }
 hashing = { path = "../../../eth2/utils/hashing" }
+int_to_bytes = { path = "../../../eth2/utils/int_to_bytes" }
 log = "0.4"
 env_logger = "0.6.0"
 rayon = "1.0"
 serde = "1.0"
 serde_derive = "1.0"
 serde_json = "1.0"
+serde_yaml = "0.8"
 slot_clock = { path = "../../../eth2/utils/slot_clock" }
 ssz = { path = "../../../eth2/utils/ssz" }
 types = { path = "../../../eth2/types" }
diff --git a/beacon_node/beacon_chain/test_harness/src/beacon_chain_harness.rs b/beacon_node/beacon_chain/test_harness/src/beacon_chain_harness.rs
index f220619ce..c442c05db 100644
--- a/beacon_node/beacon_chain/test_harness/src/beacon_chain_harness.rs
+++ b/beacon_node/beacon_chain/test_harness/src/beacon_chain_harness.rs
@@ -1,7 +1,7 @@
 use super::ValidatorHarness;
 use beacon_chain::{BeaconChain, BlockProcessingOutcome};
 pub use beacon_chain::{BeaconChainError, CheckPoint};
-use bls::{create_proof_of_possession, get_withdrawal_credentials};
+use bls::get_withdrawal_credentials;
 use db::{
     stores::{BeaconBlockStore, BeaconStateStore},
     MemoryDB,
@@ -10,10 +10,17 @@ use fork_choice::BitwiseLMDGhost;
 use log::debug;
 use rayon::prelude::*;
 use slot_clock::TestingSlotClock;
+use ssz::TreeHash;
 use std::collections::HashSet;
+use std::fs::File;
 use std::iter::FromIterator;
+use std::path::Path;
 use std::sync::Arc;
-use types::*;
+use types::{beacon_state::BeaconStateBuilder, test_utils::generate_deterministic_keypairs, *};
+
+mod generate_deposits;
+
+pub use generate_deposits::generate_deposits_from_keypairs;
 
 /// The beacon chain harness simulates a single beacon node with `validator_count` validators connected
 /// to it. Each validator is provided a borrow to the beacon chain, where it may read
@@ -35,7 +42,12 @@ impl BeaconChainHarness {
     ///
     /// - A keypair, `BlockProducer` and `Attester` for each validator.
     /// - A new BeaconChain struct where the given validators are in the genesis.
-    pub fn new(spec: ChainSpec, validator_count: usize) -> Self {
+    pub fn new(
+        spec: ChainSpec,
+        validator_count: usize,
+        validators_dir: Option<&Path>,
+        skip_deposit_verification: bool,
+    ) -> Self {
         let db = Arc::new(MemoryDB::open());
         let block_store = Arc::new(BeaconBlockStore::new(db.clone()));
         let state_store = Arc::new(BeaconStateStore::new(db.clone()));
@@ -47,50 +59,89 @@ impl BeaconChainHarness {
             block_hash: Hash256::zero(),
         };
 
-        debug!("Generating validator keypairs...");
+        let mut state_builder = BeaconStateBuilder::new(genesis_time, latest_eth1_data, &spec);
 
-        let keypairs: Vec<Keypair> = (0..validator_count)
-            .collect::<Vec<usize>>()
-            .par_iter()
-            .map(|_| Keypair::random())
-            .collect();
+        // If a `validators_dir` is specified, load the keypairs a YAML file.
+        //
+        // Otherwise, generate them deterministically where the first validator has a secret key of
+        // `1`, etc.
+        let keypairs = if let Some(path) = validators_dir {
+            debug!("Loading validator keypairs from file...");
+            let keypairs_file = File::open(path.join("keypairs.yaml")).unwrap();
+            let mut keypairs: Vec<Keypair> = serde_yaml::from_reader(&keypairs_file).unwrap();
+            keypairs.truncate(validator_count);
+            keypairs
+        } else {
+            debug!("Generating validator keypairs...");
+            generate_deterministic_keypairs(validator_count)
+        };
 
-        debug!("Creating validator deposits...");
+        // Skipping deposit verification means directly generating `Validator` records, instead
+        // of generating `Deposit` objects, verifying them and converting them into `Validator`
+        // records.
+        //
+        // It is much faster to skip deposit verification, however it does not test the initial
+        // validator induction part of beacon chain genesis.
+        if skip_deposit_verification {
+            let validators = keypairs
+                .iter()
+                .map(|keypair| {
+                    let withdrawal_credentials = Hash256::from_slice(&get_withdrawal_credentials(
+                        &keypair.pk,
+                        spec.bls_withdrawal_prefix_byte,
+                    ));
 
-        let initial_validator_deposits = keypairs
-            .par_iter()
-            .map(|keypair| Deposit {
-                branch: vec![], // branch verification is not specified.
-                index: 0,       // index verification is not specified.
-                deposit_data: DepositData {
-                    amount: 32_000_000_000, // 32 ETH (in Gwei)
-                    timestamp: genesis_time - 1,
-                    deposit_input: DepositInput {
+                    Validator {
                         pubkey: keypair.pk.clone(),
-                        // Validator can withdraw using their main keypair.
-                        withdrawal_credentials: Hash256::from_slice(
-                            &get_withdrawal_credentials(
-                                &keypair.pk,
-                                spec.bls_withdrawal_prefix_byte,
-                            )[..],
-                        ),
-                        proof_of_possession: create_proof_of_possession(&keypair),
-                    },
-                },
-            })
-            .collect();
+                        withdrawal_credentials,
+                        activation_epoch: spec.far_future_epoch,
+                        exit_epoch: spec.far_future_epoch,
+                        withdrawable_epoch: spec.far_future_epoch,
+                        initiated_exit: false,
+                        slashed: false,
+                    }
+                })
+                .collect();
 
-        debug!("Creating the BeaconChain...");
+            let balances = vec![32_000_000_000; validator_count];
+
+            state_builder.import_existing_validators(
+                validators,
+                balances,
+                validator_count as u64,
+                &spec,
+            );
+        } else {
+            debug!("Generating initial validator deposits...");
+            let deposits = generate_deposits_from_keypairs(
+                &keypairs,
+                genesis_time,
+                spec.get_domain(
+                    spec.genesis_epoch,
+                    Domain::Deposit,
+                    &Fork {
+                        previous_version: spec.genesis_fork_version,
+                        current_version: spec.genesis_fork_version,
+                        epoch: spec.genesis_epoch,
+                    },
+                ),
+                &spec,
+            );
+            state_builder.process_initial_deposits(&deposits, &spec);
+        };
+
+        let genesis_state = state_builder.build(&spec).unwrap();
+        let state_root = Hash256::from_slice(&genesis_state.hash_tree_root());
+        let genesis_block = BeaconBlock::genesis(state_root, &spec);
 
         // Create the Beacon Chain
         let beacon_chain = Arc::new(
-            BeaconChain::genesis(
+            BeaconChain::from_genesis(
                 state_store.clone(),
                 block_store.clone(),
                 slot_clock,
-                genesis_time,
-                latest_eth1_data,
-                initial_validator_deposits,
+                genesis_state,
+                genesis_block,
                 spec.clone(),
                 fork_choice,
             )
diff --git a/beacon_node/beacon_chain/test_harness/src/beacon_chain_harness/generate_deposits.rs b/beacon_node/beacon_chain/test_harness/src/beacon_chain_harness/generate_deposits.rs
new file mode 100644
index 000000000..bba3aec1c
--- /dev/null
+++ b/beacon_node/beacon_chain/test_harness/src/beacon_chain_harness/generate_deposits.rs
@@ -0,0 +1,46 @@
+use bls::get_withdrawal_credentials;
+use log::debug;
+use rayon::prelude::*;
+use types::*;
+
+/// Generates a `Deposit` for each keypairs
+pub fn generate_deposits_from_keypairs(
+    keypairs: &[Keypair],
+    genesis_time: u64,
+    domain: u64,
+    spec: &ChainSpec,
+) -> Vec<Deposit> {
+    debug!(
+        "Generating {} validator deposits from random keypairs...",
+        keypairs.len()
+    );
+
+    let initial_validator_deposits = keypairs
+        .par_iter()
+        .map(|keypair| {
+            let withdrawal_credentials = Hash256::from_slice(
+                &get_withdrawal_credentials(&keypair.pk, spec.bls_withdrawal_prefix_byte)[..],
+            );
+            Deposit {
+                branch: vec![], // branch verification is not specified.
+                index: 0,       // index verification is not specified.
+                deposit_data: DepositData {
+                    amount: 32_000_000_000, // 32 ETH (in Gwei)
+                    timestamp: genesis_time - 1,
+                    deposit_input: DepositInput {
+                        pubkey: keypair.pk.clone(),
+                        // Validator can withdraw using their main keypair.
+                        withdrawal_credentials: withdrawal_credentials.clone(),
+                        proof_of_possession: DepositInput::create_proof_of_possession(
+                            &keypair,
+                            &withdrawal_credentials,
+                            domain,
+                        ),
+                    },
+                },
+            }
+        })
+        .collect();
+
+    initial_validator_deposits
+}
diff --git a/beacon_node/beacon_chain/test_harness/src/bin.rs b/beacon_node/beacon_chain/test_harness/src/bin.rs
index 283cb0dfa..3afc921de 100644
--- a/beacon_node/beacon_chain/test_harness/src/bin.rs
+++ b/beacon_node/beacon_chain/test_harness/src/bin.rs
@@ -1,69 +1,102 @@
-use clap::{App, Arg};
+use clap::{App, Arg, SubCommand};
 use env_logger::{Builder, Env};
-use std::{fs::File, io::prelude::*};
-use test_case::TestCase;
-use yaml_rust::YamlLoader;
+use gen_keys::gen_keys;
+use run_test::run_test;
+use std::fs;
+use types::test_utils::keypairs_path;
+use types::ChainSpec;
 
 mod beacon_chain_harness;
+mod gen_keys;
+mod run_test;
 mod test_case;
 mod validator_harness;
 
 use validator_harness::ValidatorHarness;
 
 fn main() {
+    let validator_file_path = keypairs_path();
+
+    let _ = fs::create_dir(validator_file_path.parent().unwrap());
+
     let matches = App::new("Lighthouse Test Harness Runner")
         .version("0.0.1")
         .author("Sigma Prime <contact@sigmaprime.io>")
         .about("Runs `test_harness` using a YAML test_case.")
-        .arg(
-            Arg::with_name("yaml")
-                .long("yaml")
-                .value_name("FILE")
-                .help("YAML file test_case.")
-                .required(true),
-        )
         .arg(
             Arg::with_name("log")
                 .long("log-level")
+                .short("l")
                 .value_name("LOG_LEVEL")
                 .help("Logging level.")
                 .possible_values(&["error", "warn", "info", "debug", "trace"])
                 .default_value("debug")
                 .required(true),
         )
+        .arg(
+            Arg::with_name("spec")
+                .long("spec")
+                .short("s")
+                .value_name("SPECIFICATION")
+                .help("ChainSpec instantiation.")
+                .possible_values(&["foundation", "few_validators"])
+                .default_value("foundation"),
+        )
+        .subcommand(
+            SubCommand::with_name("run_test")
+                .about("Executes a YAML test specification")
+                .arg(
+                    Arg::with_name("yaml")
+                        .long("yaml")
+                        .value_name("FILE")
+                        .help("YAML file test_case.")
+                        .required(true),
+                )
+                .arg(
+                    Arg::with_name("validators_dir")
+                        .long("validators-dir")
+                        .short("v")
+                        .value_name("VALIDATORS_DIR")
+                        .help("A directory with validator deposits and keypair YAML."),
+                ),
+        )
+        .subcommand(
+            SubCommand::with_name("gen_keys")
+                .about("Builds a file of BLS keypairs for faster tests.")
+                .arg(
+                    Arg::with_name("validator_count")
+                        .long("validator_count")
+                        .short("n")
+                        .value_name("VALIDATOR_COUNT")
+                        .help("Number of validators to generate.")
+                        .required(true),
+                )
+                .arg(
+                    Arg::with_name("output_file")
+                        .long("output_file")
+                        .short("d")
+                        .value_name("GENESIS_TIME")
+                        .help("Output directory for generated YAML.")
+                        .default_value(validator_file_path.to_str().unwrap()),
+                ),
+        )
         .get_matches();
 
     if let Some(log_level) = matches.value_of("log") {
         Builder::from_env(Env::default().default_filter_or(log_level)).init();
     }
 
-    if let Some(yaml_file) = matches.value_of("yaml") {
-        let docs = {
-            let mut file = File::open(yaml_file).unwrap();
+    let _spec = match matches.value_of("spec") {
+        Some("foundation") => ChainSpec::foundation(),
+        Some("few_validators") => ChainSpec::few_validators(),
+        _ => unreachable!(), // Has a default value, should always exist.
+    };
 
-            let mut yaml_str = String::new();
-            file.read_to_string(&mut yaml_str).unwrap();
+    if let Some(matches) = matches.subcommand_matches("run_test") {
+        run_test(matches);
+    }
 
-            YamlLoader::load_from_str(&yaml_str).unwrap()
-        };
-
-        for doc in &docs {
-            // For each `test_cases` YAML in the document, build a `TestCase`, execute it and
-            // assert that the execution result matches the test_case description.
-            //
-            // In effect, for each `test_case` a new `BeaconChainHarness` is created from genesis
-            // and a new `BeaconChain` is built as per the test_case.
-            //
-            // After the `BeaconChain` has been built out as per the test_case, a dump of all blocks
-            // and states in the chain is obtained and checked against the `results` specified in
-            // the `test_case`.
-            //
-            // If any of the expectations in the results are not met, the process
-            // panics with a message.
-            for test_case in doc["test_cases"].as_vec().unwrap() {
-                let test_case = TestCase::from_yaml(test_case);
-                test_case.assert_result_valid(test_case.execute())
-            }
-        }
+    if let Some(matches) = matches.subcommand_matches("gen_keys") {
+        gen_keys(matches);
     }
 }
diff --git a/beacon_node/beacon_chain/test_harness/src/gen_keys.rs b/beacon_node/beacon_chain/test_harness/src/gen_keys.rs
new file mode 100644
index 000000000..abd512423
--- /dev/null
+++ b/beacon_node/beacon_chain/test_harness/src/gen_keys.rs
@@ -0,0 +1,21 @@
+use clap::{value_t, ArgMatches};
+use log::debug;
+use std::path::Path;
+use types::test_utils::{generate_deterministic_keypairs, KeypairsFile};
+
+/// Creates a file containing BLS keypairs.
+pub fn gen_keys(matches: &ArgMatches) {
+    let validator_count = value_t!(matches.value_of("validator_count"), usize)
+        .expect("Validator count is required argument");
+    let output_file = matches
+        .value_of("output_file")
+        .expect("Output file has a default value.");
+
+    let keypairs = generate_deterministic_keypairs(validator_count);
+
+    debug!("Writing keypairs to file...");
+
+    let keypairs_path = Path::new(output_file);
+
+    keypairs.to_raw_file(&keypairs_path, &keypairs).unwrap();
+}
diff --git a/beacon_node/beacon_chain/test_harness/src/lib.rs b/beacon_node/beacon_chain/test_harness/src/lib.rs
index 0703fd4a5..f58c1b598 100644
--- a/beacon_node/beacon_chain/test_harness/src/lib.rs
+++ b/beacon_node/beacon_chain/test_harness/src/lib.rs
@@ -15,7 +15,7 @@
 //! let validator_count = 8;
 //! let spec = ChainSpec::few_validators();
 //!
-//! let mut harness = BeaconChainHarness::new(spec, validator_count);
+//! let mut harness = BeaconChainHarness::new(spec, validator_count, None, true);
 //!
 //! harness.advance_chain_with_block();
 //!
diff --git a/beacon_node/beacon_chain/test_harness/src/run_test.rs b/beacon_node/beacon_chain/test_harness/src/run_test.rs
new file mode 100644
index 000000000..d4e2e1cf2
--- /dev/null
+++ b/beacon_node/beacon_chain/test_harness/src/run_test.rs
@@ -0,0 +1,42 @@
+use crate::test_case::TestCase;
+use clap::ArgMatches;
+use std::path::Path;
+use std::{fs::File, io::prelude::*};
+use yaml_rust::YamlLoader;
+
+/// Runs a YAML-specified test case.
+pub fn run_test(matches: &ArgMatches) {
+    if let Some(yaml_file) = matches.value_of("yaml") {
+        let docs = {
+            let mut file = File::open(yaml_file).unwrap();
+
+            let mut yaml_str = String::new();
+            file.read_to_string(&mut yaml_str).unwrap();
+
+            YamlLoader::load_from_str(&yaml_str).unwrap()
+        };
+
+        for doc in &docs {
+            let validators_dir = matches
+                .value_of("validators_dir")
+                .and_then(|dir_str| Some(Path::new(dir_str)));
+
+            // For each `test_cases` YAML in the document, build a `TestCase`, execute it and
+            // assert that the execution result matches the test_case description.
+            //
+            // In effect, for each `test_case` a new `BeaconChainHarness` is created from genesis
+            // and a new `BeaconChain` is built as per the test_case.
+            //
+            // After the `BeaconChain` has been built out as per the test_case, a dump of all blocks
+            // and states in the chain is obtained and checked against the `results` specified in
+            // the `test_case`.
+            //
+            // If any of the expectations in the results are not met, the process
+            // panics with a message.
+            for test_case in doc["test_cases"].as_vec().unwrap() {
+                let test_case = TestCase::from_yaml(test_case);
+                test_case.assert_result_valid(test_case.execute(validators_dir))
+            }
+        }
+    }
+}
diff --git a/beacon_node/beacon_chain/test_harness/src/test_case.rs b/beacon_node/beacon_chain/test_harness/src/test_case.rs
index b2709edfc..b6b1ea5cc 100644
--- a/beacon_node/beacon_chain/test_harness/src/test_case.rs
+++ b/beacon_node/beacon_chain/test_harness/src/test_case.rs
@@ -3,14 +3,13 @@
 
 use crate::beacon_chain_harness::BeaconChainHarness;
 use beacon_chain::CheckPoint;
-use bls::{create_proof_of_possession, get_withdrawal_credentials};
+use bls::get_withdrawal_credentials;
 use log::{info, warn};
 use ssz::SignedRoot;
+use std::path::Path;
 use types::*;
 
-use types::{
-    attester_slashing::AttesterSlashingBuilder, proposer_slashing::ProposerSlashingBuilder,
-};
+use types::test_utils::{TestingAttesterSlashingBuilder, TestingProposerSlashingBuilder};
 use yaml_rust::Yaml;
 
 mod config;
@@ -70,7 +69,7 @@ impl TestCase {
 
     /// Executes the test case, returning an `ExecutionResult`.
     #[allow(clippy::cyclomatic_complexity)]
-    pub fn execute(&self) -> ExecutionResult {
+    pub fn execute(&self, validators_dir: Option<&Path>) -> ExecutionResult {
         let spec = self.spec();
         let validator_count = self.config.deposits_for_chain_start;
         let slots = self.config.num_slots;
@@ -80,7 +79,7 @@ impl TestCase {
             validator_count
         );
 
-        let mut harness = BeaconChainHarness::new(spec, validator_count);
+        let mut harness = BeaconChainHarness::new(spec, validator_count, validators_dir, true);
 
         info!("Starting simulation across {} slots...", slots);
 
@@ -257,11 +256,23 @@ fn build_deposit(
     index_offset: u64,
 ) -> (Deposit, Keypair) {
     let keypair = Keypair::random();
-    let proof_of_possession = create_proof_of_possession(&keypair);
-    let index = harness.beacon_chain.state.read().deposit_index + index_offset;
     let withdrawal_credentials = Hash256::from_slice(
         &get_withdrawal_credentials(&keypair.pk, harness.spec.bls_withdrawal_prefix_byte)[..],
     );
+    let proof_of_possession = DepositInput::create_proof_of_possession(
+        &keypair,
+        &withdrawal_credentials,
+        harness.spec.get_domain(
+            harness
+                .beacon_chain
+                .state
+                .read()
+                .current_epoch(&harness.spec),
+            Domain::Deposit,
+            &harness.beacon_chain.state.read().fork,
+        ),
+    );
+    let index = harness.beacon_chain.state.read().deposit_index + index_offset;
 
     let deposit = Deposit {
         // Note: `branch` and `index` will need to be updated once the spec defines their
@@ -318,7 +329,7 @@ fn build_double_vote_attester_slashing(
             .expect("Unable to sign AttesterSlashing")
     };
 
-    AttesterSlashingBuilder::double_vote(validator_indices, signer)
+    TestingAttesterSlashingBuilder::double_vote(validator_indices, signer)
 }
 
 /// Builds an `ProposerSlashing` for some `validator_index`.
@@ -331,5 +342,5 @@ fn build_proposer_slashing(harness: &BeaconChainHarness, validator_index: u64) -
             .expect("Unable to sign AttesterSlashing")
     };
 
-    ProposerSlashingBuilder::double_vote(validator_index, signer, &harness.spec)
+    TestingProposerSlashingBuilder::double_vote(validator_index, signer, &harness.spec)
 }
diff --git a/beacon_node/beacon_chain/test_harness/tests/chain.rs b/beacon_node/beacon_chain/test_harness/tests/chain.rs
index e72c3a5aa..e5a52a314 100644
--- a/beacon_node/beacon_chain/test_harness/tests/chain.rs
+++ b/beacon_node/beacon_chain/test_harness/tests/chain.rs
@@ -10,7 +10,7 @@ fn it_can_build_on_genesis_block() {
     let spec = ChainSpec::few_validators();
     let validator_count = 8;
 
-    let mut harness = BeaconChainHarness::new(spec, validator_count as usize);
+    let mut harness = BeaconChainHarness::new(spec, validator_count as usize, None, true);
 
     harness.advance_chain_with_block();
 }
@@ -25,7 +25,7 @@ fn it_can_produce_past_first_epoch_boundary() {
 
     debug!("Starting harness build...");
 
-    let mut harness = BeaconChainHarness::new(spec, validator_count);
+    let mut harness = BeaconChainHarness::new(spec, validator_count, None, true);
 
     debug!("Harness built, tests starting..");
 
diff --git a/beacon_node/src/main.rs b/beacon_node/src/main.rs
index 072315b6b..c3182c789 100644
--- a/beacon_node/src/main.rs
+++ b/beacon_node/src/main.rs
@@ -8,7 +8,6 @@ use std::path::PathBuf;
 use crate::config::LighthouseConfig;
 use crate::rpc::start_server;
 use beacon_chain::BeaconChain;
-use bls::create_proof_of_possession;
 use clap::{App, Arg};
 use db::{
     stores::{BeaconBlockStore, BeaconStateStore},
@@ -17,8 +16,12 @@ use db::{
 use fork_choice::BitwiseLMDGhost;
 use slog::{error, info, o, Drain};
 use slot_clock::SystemTimeSlotClock;
+use ssz::TreeHash;
 use std::sync::Arc;
-use types::{ChainSpec, Deposit, DepositData, DepositInput, Eth1Data, Hash256, Keypair};
+use types::{
+    beacon_state::BeaconStateBuilder, BeaconBlock, ChainSpec, Deposit, DepositData, DepositInput,
+    Domain, Eth1Data, Fork, Hash256, Keypair,
+};
 
 fn main() {
     let decorator = slog_term::TermDecorator::new().build();
@@ -97,7 +100,8 @@ fn main() {
         .iter()
         .map(|_| Keypair::random())
         .collect();
-    let initial_validator_deposits = keypairs
+
+    let initial_validator_deposits: Vec<Deposit> = keypairs
         .iter()
         .map(|keypair| Deposit {
             branch: vec![], // branch verification is not specified.
@@ -108,20 +112,38 @@ fn main() {
                 deposit_input: DepositInput {
                     pubkey: keypair.pk.clone(),
                     withdrawal_credentials: Hash256::zero(), // Withdrawal not possible.
-                    proof_of_possession: create_proof_of_possession(&keypair),
+                    proof_of_possession: DepositInput::create_proof_of_possession(
+                        &keypair,
+                        &Hash256::zero(),
+                        spec.get_domain(
+                            // Get domain from genesis fork_version
+                            spec.genesis_epoch,
+                            Domain::Deposit,
+                            &Fork {
+                                previous_version: spec.genesis_fork_version,
+                                current_version: spec.genesis_fork_version,
+                                epoch: spec.genesis_epoch,
+                            },
+                        ),
+                    ),
                 },
             },
         })
         .collect();
 
+    let mut state_builder = BeaconStateBuilder::new(genesis_time, latest_eth1_data, &spec);
+    state_builder.process_initial_deposits(&initial_validator_deposits, &spec);
+    let genesis_state = state_builder.build(&spec).unwrap();
+    let state_root = Hash256::from_slice(&genesis_state.hash_tree_root());
+    let genesis_block = BeaconBlock::genesis(state_root, &spec);
+
     // Genesis chain
-    let _chain_result = BeaconChain::genesis(
+    let _chain_result = BeaconChain::from_genesis(
         state_store.clone(),
         block_store.clone(),
         slot_clock,
-        genesis_time,
-        latest_eth1_data,
-        initial_validator_deposits,
+        genesis_state,
+        genesis_block,
         spec,
         fork_choice,
     );
diff --git a/eth2/fork_choice/tests/tests.rs b/eth2/fork_choice/tests/tests.rs
index a3cab6a7c..7228bca10 100644
--- a/eth2/fork_choice/tests/tests.rs
+++ b/eth2/fork_choice/tests/tests.rs
@@ -12,7 +12,7 @@ extern crate types;
 extern crate yaml_rust;
 
 pub use beacon_chain::BeaconChain;
-use bls::{PublicKey, Signature};
+use bls::Signature;
 use db::stores::{BeaconBlockStore, BeaconStateStore};
 use db::MemoryDB;
 //use env_logger::{Builder, Env};
@@ -21,9 +21,8 @@ use ssz::ssz_encode;
 use std::collections::HashMap;
 use std::sync::Arc;
 use std::{fs::File, io::prelude::*, path::PathBuf};
-use types::{
-    BeaconBlock, BeaconBlockBody, BeaconState, ChainSpec, Epoch, Eth1Data, Hash256, Slot, Validator,
-};
+use types::test_utils::TestingBeaconStateBuilder;
+use types::{BeaconBlock, BeaconBlockBody, ChainSpec, Eth1Data, Hash256, Slot};
 use yaml_rust::yaml;
 
 // Note: We Assume the block Id's are hex-encoded.
@@ -207,8 +206,6 @@ fn setup_inital_state(
     fork_choice_algo: &ForkChoiceAlgorithm,
     no_validators: usize,
 ) -> (Box<ForkChoice>, Arc<BeaconBlockStore<MemoryDB>>, Hash256) {
-    let zero_hash = Hash256::zero();
-
     let db = Arc::new(MemoryDB::open());
     let block_store = Arc::new(BeaconBlockStore::new(db.clone()));
     let state_store = Arc::new(BeaconStateStore::new(db.clone()));
@@ -225,40 +222,11 @@ fn setup_inital_state(
         ForkChoiceAlgorithm::LongestChain => Box::new(LongestChain::new(block_store.clone())),
     };
 
-    // misc vars for setting up the state
-    let genesis_time = 1_550_381_159;
-
-    let latest_eth1_data = Eth1Data {
-        deposit_root: zero_hash.clone(),
-        block_hash: zero_hash.clone(),
-    };
-
-    let initial_validator_deposits = vec![];
     let spec = ChainSpec::foundation();
 
-    // create the state
-    let mut state = BeaconState::genesis(
-        genesis_time,
-        initial_validator_deposits,
-        latest_eth1_data,
-        &spec,
-    )
-    .unwrap();
-
-    let default_validator = Validator {
-        pubkey: PublicKey::default(),
-        withdrawal_credentials: zero_hash,
-        activation_epoch: Epoch::from(0u64),
-        exit_epoch: spec.far_future_epoch,
-        withdrawable_epoch: spec.far_future_epoch,
-        initiated_exit: false,
-        slashed: false,
-    };
-    // activate the validators
-    for _ in 0..no_validators {
-        state.validator_registry.push(default_validator.clone());
-        state.validator_balances.push(32_000_000_000);
-    }
+    let state_builder =
+        TestingBeaconStateBuilder::from_deterministic_keypairs(no_validators, &spec);
+    let (state, _keypairs) = state_builder.build();
 
     let state_root = state.canonical_root();
     state_store
diff --git a/eth2/state_processing/Cargo.toml b/eth2/state_processing/Cargo.toml
index c51ce8372..f6692b259 100644
--- a/eth2/state_processing/Cargo.toml
+++ b/eth2/state_processing/Cargo.toml
@@ -14,6 +14,7 @@ env_logger = "0.6.0"
 
 [dependencies]
 bls = { path = "../utils/bls" }
+fnv = "1.0"
 hashing = { path = "../utils/hashing" }
 int_to_bytes = { path = "../utils/int_to_bytes" }
 integer-sqrt = "0.1"
diff --git a/eth2/state_processing/benches/bench_block_processing.rs b/eth2/state_processing/benches/bench_block_processing.rs
new file mode 100644
index 000000000..031942473
--- /dev/null
+++ b/eth2/state_processing/benches/bench_block_processing.rs
@@ -0,0 +1,437 @@
+use criterion::Criterion;
+use criterion::{black_box, Benchmark};
+use log::debug;
+use ssz::TreeHash;
+use state_processing::{
+    per_block_processing,
+    per_block_processing::{
+        process_attestations, process_attester_slashings, process_deposits, process_eth1_data,
+        process_exits, process_proposer_slashings, process_randao, process_transfers,
+        verify_block_signature,
+    },
+};
+use types::test_utils::{TestingBeaconBlockBuilder, TestingBeaconStateBuilder};
+use types::*;
+
+/// Run the benchmarking suite on a foundation spec with 16,384 validators.
+pub fn bench_block_processing_n_validators(c: &mut Criterion, validator_count: usize) {
+    let spec = ChainSpec::foundation();
+
+    let (mut state, keypairs) = build_state(validator_count, &spec);
+    let block = build_block(&mut state, &keypairs, &spec);
+
+    assert_eq!(
+        block.body.proposer_slashings.len(),
+        spec.max_proposer_slashings as usize,
+        "The block should have the maximum possible proposer slashings"
+    );
+
+    assert_eq!(
+        block.body.attester_slashings.len(),
+        spec.max_attester_slashings as usize,
+        "The block should have the maximum possible attester slashings"
+    );
+
+    for attester_slashing in &block.body.attester_slashings {
+        let len_1 = attester_slashing
+            .slashable_attestation_1
+            .validator_indices
+            .len();
+        let len_2 = attester_slashing
+            .slashable_attestation_1
+            .validator_indices
+            .len();
+        assert!(
+            (len_1 == len_2) && (len_2 == spec.max_indices_per_slashable_vote as usize),
+            "Each attester slashing should have the maximum possible validator indices"
+        );
+    }
+
+    assert_eq!(
+        block.body.attestations.len(),
+        spec.max_attestations as usize,
+        "The block should have the maximum possible attestations."
+    );
+
+    assert_eq!(
+        block.body.deposits.len(),
+        spec.max_deposits as usize,
+        "The block should have the maximum possible deposits."
+    );
+
+    assert_eq!(
+        block.body.voluntary_exits.len(),
+        spec.max_voluntary_exits as usize,
+        "The block should have the maximum possible voluntary exits."
+    );
+
+    assert_eq!(
+        block.body.transfers.len(),
+        spec.max_transfers as usize,
+        "The block should have the maximum possible transfers."
+    );
+
+    bench_block_processing(
+        c,
+        &block,
+        &state,
+        &spec,
+        &format!("{}_validators", validator_count),
+    );
+}
+
+fn build_state(validator_count: usize, spec: &ChainSpec) -> (BeaconState, Vec<Keypair>) {
+    let mut builder =
+        TestingBeaconStateBuilder::from_default_keypairs_file_if_exists(validator_count, &spec);
+
+    // Set the state to be just before an epoch transition.
+    let target_slot = (spec.genesis_epoch + 4).end_slot(spec.slots_per_epoch);
+    builder.teleport_to_slot(target_slot, &spec);
+
+    // Builds all caches; benches will not contain shuffling/committee building times.
+    builder.build_caches(&spec).unwrap();
+
+    builder.build()
+}
+
+fn build_block(state: &mut BeaconState, keypairs: &[Keypair], spec: &ChainSpec) -> BeaconBlock {
+    let mut builder = TestingBeaconBlockBuilder::new(spec);
+
+    builder.set_slot(state.slot);
+
+    let proposer_index = state.get_beacon_proposer_index(state.slot, spec).unwrap();
+    let keypair = &keypairs[proposer_index];
+
+    builder.set_randao_reveal(&keypair.sk, &state.fork, spec);
+
+    // Used as a stream of validator indices for use in slashings, exits, etc.
+    let mut validators_iter = (0..keypairs.len() as u64).into_iter();
+
+    // Insert the maximum possible number of `ProposerSlashing` objects.
+    debug!(
+        "Inserting {} proposer slashings...",
+        spec.max_proposer_slashings
+    );
+    for _ in 0..spec.max_proposer_slashings {
+        let validator_index = validators_iter.next().expect("Insufficient validators.");
+
+        builder.insert_proposer_slashing(
+            validator_index,
+            &keypairs[validator_index as usize].sk,
+            &state.fork,
+            spec,
+        );
+    }
+
+    // Insert the maximum possible number of `AttesterSlashing` objects
+    debug!(
+        "Inserting {} attester slashings...",
+        spec.max_attester_slashings
+    );
+    for _ in 0..spec.max_attester_slashings {
+        let mut attesters: Vec<u64> = vec![];
+        let mut secret_keys: Vec<&SecretKey> = vec![];
+
+        for _ in 0..spec.max_indices_per_slashable_vote {
+            let validator_index = validators_iter.next().expect("Insufficient validators.");
+
+            attesters.push(validator_index);
+            secret_keys.push(&keypairs[validator_index as usize].sk);
+        }
+
+        builder.insert_attester_slashing(&attesters, &secret_keys, &state.fork, spec);
+    }
+
+    // Insert the maximum possible number of `Attestation` objects.
+    debug!("Inserting {} attestations...", spec.max_attestations);
+    let all_secret_keys: Vec<&SecretKey> = keypairs.iter().map(|keypair| &keypair.sk).collect();
+    builder
+        .fill_with_attestations(state, &all_secret_keys, spec)
+        .unwrap();
+
+    // Insert the maximum possible number of `Deposit` objects.
+    debug!("Inserting {} deposits...", spec.max_deposits);
+    for i in 0..spec.max_deposits {
+        builder.insert_deposit(32_000_000_000, state.deposit_index + i, state, spec);
+    }
+
+    // Insert the maximum possible number of `Exit` objects.
+    debug!("Inserting {} exits...", spec.max_voluntary_exits);
+    for _ in 0..spec.max_voluntary_exits {
+        let validator_index = validators_iter.next().expect("Insufficient validators.");
+
+        builder.insert_exit(
+            state,
+            validator_index,
+            &keypairs[validator_index as usize].sk,
+            spec,
+        );
+    }
+
+    // Insert the maximum possible number of `Transfer` objects.
+    debug!("Inserting {} transfers...", spec.max_transfers);
+    for _ in 0..spec.max_transfers {
+        let validator_index = validators_iter.next().expect("Insufficient validators.");
+
+        // Manually set the validator to be withdrawn.
+        state.validator_registry[validator_index as usize].withdrawable_epoch =
+            state.previous_epoch(spec);
+
+        builder.insert_transfer(
+            state,
+            validator_index,
+            validator_index,
+            1,
+            keypairs[validator_index as usize].clone(),
+            spec,
+        );
+    }
+
+    let mut block = builder.build(&keypair.sk, &state.fork, spec);
+
+    // Set the eth1 data to be different from the state.
+    block.eth1_data.block_hash = Hash256::from_slice(&vec![42; 32]);
+
+    block
+}
+
+/// Run the detailed benchmarking suite on the given `BeaconState`.
+///
+/// `desc` will be added to the title of each bench.
+fn bench_block_processing(
+    c: &mut Criterion,
+    initial_block: &BeaconBlock,
+    initial_state: &BeaconState,
+    initial_spec: &ChainSpec,
+    desc: &str,
+) {
+    let state = initial_state.clone();
+    let block = initial_block.clone();
+    let spec = initial_spec.clone();
+    c.bench(
+        &format!("{}/block_processing", desc),
+        Benchmark::new("verify_block_signature", move |b| {
+            b.iter_batched(
+                || state.clone(),
+                |mut state| {
+                    verify_block_signature(&mut state, &block, &spec).unwrap();
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(10),
+    );
+
+    let state = initial_state.clone();
+    let block = initial_block.clone();
+    let spec = initial_spec.clone();
+    c.bench(
+        &format!("{}/block_processing", desc),
+        Benchmark::new("process_randao", move |b| {
+            b.iter_batched(
+                || state.clone(),
+                |mut state| {
+                    process_randao(&mut state, &block, &spec).unwrap();
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(10),
+    );
+
+    let state = initial_state.clone();
+    let block = initial_block.clone();
+    c.bench(
+        &format!("{}/block_processing", desc),
+        Benchmark::new("process_eth1_data", move |b| {
+            b.iter_batched(
+                || state.clone(),
+                |mut state| {
+                    process_eth1_data(&mut state, &block.eth1_data).unwrap();
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(10),
+    );
+
+    let state = initial_state.clone();
+    let block = initial_block.clone();
+    let spec = initial_spec.clone();
+    c.bench(
+        &format!("{}/block_processing", desc),
+        Benchmark::new("process_proposer_slashings", move |b| {
+            b.iter_batched(
+                || state.clone(),
+                |mut state| {
+                    process_proposer_slashings(&mut state, &block.body.proposer_slashings, &spec)
+                        .unwrap();
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(10),
+    );
+
+    let state = initial_state.clone();
+    let block = initial_block.clone();
+    let spec = initial_spec.clone();
+    c.bench(
+        &format!("{}/block_processing", desc),
+        Benchmark::new("process_attester_slashings", move |b| {
+            b.iter_batched(
+                || state.clone(),
+                |mut state| {
+                    process_attester_slashings(&mut state, &block.body.attester_slashings, &spec)
+                        .unwrap();
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(10),
+    );
+
+    let state = initial_state.clone();
+    let block = initial_block.clone();
+    let spec = initial_spec.clone();
+    c.bench(
+        &format!("{}/block_processing", desc),
+        Benchmark::new("process_attestations", move |b| {
+            b.iter_batched(
+                || state.clone(),
+                |mut state| {
+                    process_attestations(&mut state, &block.body.attestations, &spec).unwrap();
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(10),
+    );
+
+    let state = initial_state.clone();
+    let block = initial_block.clone();
+    let spec = initial_spec.clone();
+    c.bench(
+        &format!("{}/block_processing", desc),
+        Benchmark::new("process_deposits", move |b| {
+            b.iter_batched(
+                || state.clone(),
+                |mut state| {
+                    process_deposits(&mut state, &block.body.deposits, &spec).unwrap();
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(10),
+    );
+
+    let state = initial_state.clone();
+    let block = initial_block.clone();
+    let spec = initial_spec.clone();
+    c.bench(
+        &format!("{}/block_processing", desc),
+        Benchmark::new("process_exits", move |b| {
+            b.iter_batched(
+                || state.clone(),
+                |mut state| {
+                    process_exits(&mut state, &block.body.voluntary_exits, &spec).unwrap();
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(10),
+    );
+
+    let state = initial_state.clone();
+    let block = initial_block.clone();
+    let spec = initial_spec.clone();
+    c.bench(
+        &format!("{}/block_processing", desc),
+        Benchmark::new("process_transfers", move |b| {
+            b.iter_batched(
+                || state.clone(),
+                |mut state| {
+                    process_transfers(&mut state, &block.body.transfers, &spec).unwrap();
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(10),
+    );
+
+    let state = initial_state.clone();
+    let block = initial_block.clone();
+    let spec = initial_spec.clone();
+    c.bench(
+        &format!("{}/block_processing", desc),
+        Benchmark::new("per_block_processing", move |b| {
+            b.iter_batched(
+                || state.clone(),
+                |mut state| {
+                    per_block_processing(&mut state, &block, &spec).unwrap();
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(10),
+    );
+
+    let mut state = initial_state.clone();
+    state.drop_cache(RelativeEpoch::Previous);
+    let spec = initial_spec.clone();
+    c.bench(
+        &format!("{}/block_processing", desc),
+        Benchmark::new("build_previous_state_epoch_cache", move |b| {
+            b.iter_batched(
+                || state.clone(),
+                |mut state| {
+                    state
+                        .build_epoch_cache(RelativeEpoch::Previous, &spec)
+                        .unwrap();
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(10),
+    );
+
+    let mut state = initial_state.clone();
+    state.drop_cache(RelativeEpoch::Current);
+    let spec = initial_spec.clone();
+    c.bench(
+        &format!("{}/block_processing", desc),
+        Benchmark::new("build_current_state_epoch_cache", move |b| {
+            b.iter_batched(
+                || state.clone(),
+                |mut state| {
+                    state
+                        .build_epoch_cache(RelativeEpoch::Current, &spec)
+                        .unwrap();
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(10),
+    );
+
+    let block = initial_block.clone();
+    c.bench(
+        &format!("{}/block_processing", desc),
+        Benchmark::new("tree_hash_block", move |b| {
+            b.iter(|| black_box(block.hash_tree_root()))
+        })
+        .sample_size(10),
+    );
+}
diff --git a/eth2/state_processing/benches/bench_epoch_processing.rs b/eth2/state_processing/benches/bench_epoch_processing.rs
new file mode 100644
index 000000000..e4981b200
--- /dev/null
+++ b/eth2/state_processing/benches/bench_epoch_processing.rs
@@ -0,0 +1,378 @@
+use criterion::Criterion;
+use criterion::{black_box, Benchmark};
+use ssz::TreeHash;
+use state_processing::{
+    per_epoch_processing,
+    per_epoch_processing::{
+        calculate_active_validator_indices, calculate_attester_sets, clean_attestations,
+        process_crosslinks, process_eth1_data, process_justification,
+        process_rewards_and_penalities, process_validator_registry, update_active_tree_index_roots,
+        update_latest_slashed_balances,
+    },
+};
+use types::test_utils::TestingBeaconStateBuilder;
+use types::{validator_registry::get_active_validator_indices, *};
+
+pub const BENCHING_SAMPLE_SIZE: usize = 10;
+pub const SMALL_BENCHING_SAMPLE_SIZE: usize = 10;
+
+/// Run the benchmarking suite on a foundation spec with 16,384 validators.
+pub fn bench_epoch_processing_n_validators(c: &mut Criterion, validator_count: usize) {
+    let spec = ChainSpec::foundation();
+
+    let mut builder =
+        TestingBeaconStateBuilder::from_default_keypairs_file_if_exists(validator_count, &spec);
+
+    // Set the state to be just before an epoch transition.
+    let target_slot = (spec.genesis_epoch + 4).end_slot(spec.slots_per_epoch);
+    builder.teleport_to_slot(target_slot, &spec);
+
+    // Builds all caches; benches will not contain shuffling/committee building times.
+    builder.build_caches(&spec).unwrap();
+
+    // Inserts one attestation with full participation for each committee able to include an
+    // attestation in this state.
+    builder.insert_attestations(&spec);
+
+    let (state, _keypairs) = builder.build();
+
+    // Assert that the state has an attestations for each committee that is able to include an
+    // attestation in the state.
+    let committees_per_epoch = spec.get_epoch_committee_count(validator_count);
+    let committees_per_slot = committees_per_epoch / spec.slots_per_epoch;
+    let previous_epoch_attestations = committees_per_epoch;
+    let current_epoch_attestations =
+        committees_per_slot * (spec.slots_per_epoch - spec.min_attestation_inclusion_delay);
+    assert_eq!(
+        state.latest_attestations.len() as u64,
+        previous_epoch_attestations + current_epoch_attestations,
+        "The state should have an attestation for each committee."
+    );
+
+    // Assert that each attestation in the state has full participation.
+    let committee_size = validator_count / committees_per_epoch as usize;
+    for a in &state.latest_attestations {
+        assert_eq!(
+            a.aggregation_bitfield.num_set_bits(),
+            committee_size,
+            "Each attestation in the state should have full participation"
+        );
+    }
+
+    // Assert that we will run the first arm of process_rewards_and_penalities
+    let epochs_since_finality = state.next_epoch(&spec) - state.finalized_epoch;
+    assert_eq!(
+        epochs_since_finality, 4,
+        "Epochs since finality should be 4"
+    );
+
+    bench_epoch_processing(c, &state, &spec, &format!("{}_validators", validator_count));
+}
+
+/// Run the detailed benchmarking suite on the given `BeaconState`.
+///
+/// `desc` will be added to the title of each bench.
+fn bench_epoch_processing(c: &mut Criterion, state: &BeaconState, spec: &ChainSpec, desc: &str) {
+    let state_clone = state.clone();
+    let spec_clone = spec.clone();
+    c.bench(
+        &format!("{}/epoch_processing", desc),
+        Benchmark::new("calculate_active_validator_indices", move |b| {
+            b.iter_batched(
+                || state_clone.clone(),
+                |mut state| {
+                    calculate_active_validator_indices(&mut state, &spec_clone);
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(BENCHING_SAMPLE_SIZE),
+    );
+
+    let state_clone = state.clone();
+    let spec_clone = spec.clone();
+    let active_validator_indices = calculate_active_validator_indices(&state, &spec);
+    c.bench(
+        &format!("{}/epoch_processing", desc),
+        Benchmark::new("calculate_current_total_balance", move |b| {
+            b.iter_batched(
+                || state_clone.clone(),
+                |state| {
+                    state.get_total_balance(&active_validator_indices[..], &spec_clone);
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(BENCHING_SAMPLE_SIZE),
+    );
+
+    let state_clone = state.clone();
+    let spec_clone = spec.clone();
+    c.bench(
+        &format!("{}/epoch_processing", desc),
+        Benchmark::new("calculate_previous_total_balance", move |b| {
+            b.iter_batched(
+                || state_clone.clone(),
+                |state| {
+                    state.get_total_balance(
+                        &get_active_validator_indices(
+                            &state.validator_registry,
+                            state.previous_epoch(&spec_clone),
+                        )[..],
+                        &spec_clone,
+                    );
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(BENCHING_SAMPLE_SIZE),
+    );
+
+    let state_clone = state.clone();
+    let spec_clone = spec.clone();
+    c.bench(
+        &format!("{}/epoch_processing", desc),
+        Benchmark::new("process_eth1_data", move |b| {
+            b.iter_batched(
+                || state_clone.clone(),
+                |mut state| {
+                    process_eth1_data(&mut state, &spec_clone);
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(BENCHING_SAMPLE_SIZE),
+    );
+
+    let state_clone = state.clone();
+    let spec_clone = spec.clone();
+    c.bench(
+        &format!("{}/epoch_processing", desc),
+        Benchmark::new("calculate_attester_sets", move |b| {
+            b.iter_batched(
+                || state_clone.clone(),
+                |mut state| {
+                    calculate_attester_sets(&mut state, &spec_clone).unwrap();
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(BENCHING_SAMPLE_SIZE),
+    );
+
+    let state_clone = state.clone();
+    let spec_clone = spec.clone();
+    let previous_epoch = state.previous_epoch(&spec);
+    let attesters = calculate_attester_sets(&state, &spec).unwrap();
+    let active_validator_indices = calculate_active_validator_indices(&state, &spec);
+    let current_total_balance = state.get_total_balance(&active_validator_indices[..], &spec);
+    let previous_total_balance = state.get_total_balance(
+        &get_active_validator_indices(&state.validator_registry, previous_epoch)[..],
+        &spec,
+    );
+    c.bench(
+        &format!("{}/epoch_processing", desc),
+        Benchmark::new("process_justification", move |b| {
+            b.iter_batched(
+                || state_clone.clone(),
+                |mut state| {
+                    process_justification(
+                        &mut state,
+                        current_total_balance,
+                        previous_total_balance,
+                        attesters.previous_epoch_boundary.balance,
+                        attesters.current_epoch_boundary.balance,
+                        &spec_clone,
+                    );
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(10),
+    );
+
+    let state_clone = state.clone();
+    let spec_clone = spec.clone();
+    c.bench(
+        &format!("{}/epoch_processing", desc),
+        Benchmark::new("process_crosslinks", move |b| {
+            b.iter_batched(
+                || state_clone.clone(),
+                |mut state| black_box(process_crosslinks(&mut state, &spec_clone).unwrap()),
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(BENCHING_SAMPLE_SIZE),
+    );
+
+    let mut state_clone = state.clone();
+    let spec_clone = spec.clone();
+    let previous_epoch = state.previous_epoch(&spec);
+    let attesters = calculate_attester_sets(&state, &spec).unwrap();
+    let active_validator_indices = calculate_active_validator_indices(&state, &spec);
+    let previous_total_balance = state.get_total_balance(
+        &get_active_validator_indices(&state.validator_registry, previous_epoch)[..],
+        &spec,
+    );
+    let winning_root_for_shards = process_crosslinks(&mut state_clone, &spec).unwrap();
+    c.bench(
+        &format!("{}/epoch_processing", desc),
+        Benchmark::new("process_rewards_and_penalties", move |b| {
+            b.iter_batched(
+                || state_clone.clone(),
+                |mut state| {
+                    process_rewards_and_penalities(
+                        &mut state,
+                        &active_validator_indices,
+                        &attesters,
+                        previous_total_balance,
+                        &winning_root_for_shards,
+                        &spec_clone,
+                    )
+                    .unwrap();
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(SMALL_BENCHING_SAMPLE_SIZE),
+    );
+
+    let state_clone = state.clone();
+    let spec_clone = spec.clone();
+    c.bench(
+        &format!("{}/epoch_processing", desc),
+        Benchmark::new("process_ejections", move |b| {
+            b.iter_batched(
+                || state_clone.clone(),
+                |mut state| {
+                    state.process_ejections(&spec_clone);
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(BENCHING_SAMPLE_SIZE),
+    );
+
+    let mut state_clone = state.clone();
+    let spec_clone = spec.clone();
+    let previous_epoch = state.previous_epoch(&spec);
+    let attesters = calculate_attester_sets(&state, &spec).unwrap();
+    let active_validator_indices = calculate_active_validator_indices(&state, &spec);
+    let current_total_balance = state.get_total_balance(&active_validator_indices[..], spec);
+    let previous_total_balance = state.get_total_balance(
+        &get_active_validator_indices(&state.validator_registry, previous_epoch)[..],
+        &spec,
+    );
+    assert_eq!(
+        state_clone.finalized_epoch, state_clone.validator_registry_update_epoch,
+        "The last registry update should be at the last finalized epoch."
+    );
+    process_justification(
+        &mut state_clone,
+        current_total_balance,
+        previous_total_balance,
+        attesters.previous_epoch_boundary.balance,
+        attesters.current_epoch_boundary.balance,
+        spec,
+    );
+    assert!(
+        state_clone.finalized_epoch > state_clone.validator_registry_update_epoch,
+        "The state should have been finalized."
+    );
+    c.bench(
+        &format!("{}/epoch_processing", desc),
+        Benchmark::new("process_validator_registry", move |b| {
+            b.iter_batched(
+                || state_clone.clone(),
+                |mut state| {
+                    process_validator_registry(&mut state, &spec_clone).unwrap();
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(BENCHING_SAMPLE_SIZE),
+    );
+
+    let state_clone = state.clone();
+    let spec_clone = spec.clone();
+    c.bench(
+        &format!("{}/epoch_processing", desc),
+        Benchmark::new("update_active_tree_index_roots", move |b| {
+            b.iter_batched(
+                || state_clone.clone(),
+                |mut state| {
+                    update_active_tree_index_roots(&mut state, &spec_clone).unwrap();
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(BENCHING_SAMPLE_SIZE),
+    );
+
+    let state_clone = state.clone();
+    let spec_clone = spec.clone();
+    c.bench(
+        &format!("{}/epoch_processing", desc),
+        Benchmark::new("update_latest_slashed_balances", move |b| {
+            b.iter_batched(
+                || state_clone.clone(),
+                |mut state| {
+                    update_latest_slashed_balances(&mut state, &spec_clone);
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(BENCHING_SAMPLE_SIZE),
+    );
+
+    let state_clone = state.clone();
+    let spec_clone = spec.clone();
+    c.bench(
+        &format!("{}/epoch_processing", desc),
+        Benchmark::new("clean_attestations", move |b| {
+            b.iter_batched(
+                || state_clone.clone(),
+                |mut state| {
+                    clean_attestations(&mut state, &spec_clone);
+                    state
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(BENCHING_SAMPLE_SIZE),
+    );
+
+    let state_clone = state.clone();
+    let spec_clone = spec.clone();
+    c.bench(
+        &format!("{}/epoch_processing", desc),
+        Benchmark::new("per_epoch_processing", move |b| {
+            b.iter_batched(
+                || state_clone.clone(),
+                |mut state| black_box(per_epoch_processing(&mut state, &spec_clone).unwrap()),
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(SMALL_BENCHING_SAMPLE_SIZE),
+    );
+
+    let state_clone = state.clone();
+    c.bench(
+        &format!("{}/epoch_processing", desc),
+        Benchmark::new("tree_hash_state", move |b| {
+            b.iter(|| black_box(state_clone.hash_tree_root()))
+        })
+        .sample_size(SMALL_BENCHING_SAMPLE_SIZE),
+    );
+}
diff --git a/eth2/state_processing/benches/benches.rs b/eth2/state_processing/benches/benches.rs
index 682259eef..ad8c4f714 100644
--- a/eth2/state_processing/benches/benches.rs
+++ b/eth2/state_processing/benches/benches.rs
@@ -1,65 +1,26 @@
+use criterion::Benchmark;
 use criterion::Criterion;
-use criterion::{black_box, criterion_group, criterion_main, Benchmark};
-// use env_logger::{Builder, Env};
-use state_processing::SlotProcessable;
-use types::beacon_state::BeaconStateBuilder;
+use criterion::{criterion_group, criterion_main};
+use env_logger::{Builder, Env};
+use types::test_utils::TestingBeaconStateBuilder;
 use types::*;
 
-fn epoch_processing(c: &mut Criterion) {
-    // Builder::from_env(Env::default().default_filter_or("debug")).init();
+mod bench_block_processing;
+mod bench_epoch_processing;
 
-    let mut builder = BeaconStateBuilder::new(16_384);
+pub const VALIDATOR_COUNT: usize = 300_032;
 
-    builder.build_fast().unwrap();
-    builder.teleport_to_end_of_epoch(builder.spec.genesis_epoch + 4);
+// `LOG_LEVEL == "debug"` gives logs, but they're very noisy and slow down benching.
+pub const LOG_LEVEL: &str = "";
 
-    let mut state = builder.cloned_state();
+pub fn state_processing(c: &mut Criterion) {
+    if LOG_LEVEL != "" {
+        Builder::from_env(Env::default().default_filter_or(LOG_LEVEL)).init();
+    }
 
-    // Build all the caches so the following state does _not_ include the cache-building time.
-    state
-        .build_epoch_cache(RelativeEpoch::Previous, &builder.spec)
-        .unwrap();
-    state
-        .build_epoch_cache(RelativeEpoch::Current, &builder.spec)
-        .unwrap();
-    state
-        .build_epoch_cache(RelativeEpoch::Next, &builder.spec)
-        .unwrap();
-
-    let cached_state = state.clone();
-
-    // Drop all the caches so the following state includes the cache-building time.
-    state.drop_cache(RelativeEpoch::Previous);
-    state.drop_cache(RelativeEpoch::Current);
-    state.drop_cache(RelativeEpoch::Next);
-
-    let cacheless_state = state;
-
-    let spec_a = builder.spec.clone();
-    let spec_b = builder.spec.clone();
-
-    c.bench(
-        "epoch processing",
-        Benchmark::new("with pre-built caches", move |b| {
-            b.iter_with_setup(
-                || cached_state.clone(),
-                |mut state| black_box(state.per_slot_processing(Hash256::zero(), &spec_a).unwrap()),
-            )
-        })
-        .sample_size(10),
-    );
-
-    c.bench(
-        "epoch processing",
-        Benchmark::new("without pre-built caches", move |b| {
-            b.iter_with_setup(
-                || cacheless_state.clone(),
-                |mut state| black_box(state.per_slot_processing(Hash256::zero(), &spec_b).unwrap()),
-            )
-        })
-        .sample_size(10),
-    );
+    bench_block_processing::bench_block_processing_n_validators(c, VALIDATOR_COUNT);
+    bench_epoch_processing::bench_epoch_processing_n_validators(c, VALIDATOR_COUNT);
 }
 
-criterion_group!(benches, epoch_processing,);
+criterion_group!(benches, state_processing);
 criterion_main!(benches);
diff --git a/eth2/state_processing/src/per_block_processing.rs b/eth2/state_processing/src/per_block_processing.rs
index 1ab1eed71..7b5aafa7f 100644
--- a/eth2/state_processing/src/per_block_processing.rs
+++ b/eth2/state_processing/src/per_block_processing.rs
@@ -1,14 +1,19 @@
 use self::verify_proposer_slashing::verify_proposer_slashing;
 use errors::{BlockInvalid as Invalid, BlockProcessingError as Error, IntoWithIndex};
 use hashing::hash;
-use log::debug;
+use rayon::prelude::*;
 use ssz::{ssz_encode, SignedRoot, TreeHash};
 use types::*;
 
-pub use self::verify_attester_slashing::verify_attester_slashing;
+pub use self::verify_attester_slashing::{
+    gather_attester_slashing_indices, verify_attester_slashing,
+};
 pub use validate_attestation::{validate_attestation, validate_attestation_without_signature};
-pub use verify_deposit::verify_deposit;
+pub use verify_deposit::{
+    build_public_key_hashmap, get_existing_validator_index, verify_deposit, verify_deposit_index,
+};
 pub use verify_exit::verify_exit;
+pub use verify_slashable_attestation::verify_slashable_attestation;
 pub use verify_transfer::{execute_transfer, verify_transfer};
 
 pub mod errors;
@@ -70,22 +75,21 @@ fn per_block_processing_signature_optional(
     // Verify that `block.slot == state.slot`.
     verify!(block.slot == state.slot, Invalid::StateSlotMismatch);
 
-    // Ensure the current epoch cache is built.
+    // Ensure the current and previous epoch cache is built.
     state.build_epoch_cache(RelativeEpoch::Current, spec)?;
+    state.build_epoch_cache(RelativeEpoch::Previous, spec)?;
 
     if should_verify_block_signature {
         verify_block_signature(&state, &block, &spec)?;
     }
     process_randao(&mut state, &block, &spec)?;
     process_eth1_data(&mut state, &block.eth1_data)?;
-    process_proposer_slashings(&mut state, &block.body.proposer_slashings[..], spec)?;
-    process_attester_slashings(&mut state, &block.body.attester_slashings[..], spec)?;
-    process_attestations(&mut state, &block.body.attestations[..], spec)?;
-    process_deposits(&mut state, &block.body.deposits[..], spec)?;
-    process_exits(&mut state, &block.body.voluntary_exits[..], spec)?;
-    process_transfers(&mut state, &block.body.transfers[..], spec)?;
-
-    debug!("per_block_processing complete.");
+    process_proposer_slashings(&mut state, &block.body.proposer_slashings, spec)?;
+    process_attester_slashings(&mut state, &block.body.attester_slashings, spec)?;
+    process_attestations(&mut state, &block.body.attestations, spec)?;
+    process_deposits(&mut state, &block.body.deposits, spec)?;
+    process_exits(&mut state, &block.body.voluntary_exits, spec)?;
+    process_transfers(&mut state, &block.body.transfers, spec)?;
 
     Ok(())
 }
@@ -228,9 +232,17 @@ pub fn process_proposer_slashings(
         proposer_slashings.len() as u64 <= spec.max_proposer_slashings,
         Invalid::MaxProposerSlashingsExceeded
     );
-    for (i, proposer_slashing) in proposer_slashings.iter().enumerate() {
-        verify_proposer_slashing(proposer_slashing, &state, spec)
-            .map_err(|e| e.into_with_index(i))?;
+
+    // Verify proposer slashings in parallel.
+    proposer_slashings
+        .par_iter()
+        .enumerate()
+        .try_for_each(|(i, proposer_slashing)| {
+            verify_proposer_slashing(proposer_slashing, &state, spec)
+                .map_err(|e| e.into_with_index(i))
+        })?;
+
+    for proposer_slashing in proposer_slashings {
         state.slash_validator(proposer_slashing.proposer_index as usize, spec)?;
     }
 
@@ -252,9 +264,41 @@ pub fn process_attester_slashings(
         attester_slashings.len() as u64 <= spec.max_attester_slashings,
         Invalid::MaxAttesterSlashingsExceed
     );
+
+    // Verify the `SlashableAttestation`s in parallel (these are the resource-consuming objects, not
+    // the `AttesterSlashing`s themselves).
+    let mut slashable_attestations: Vec<&SlashableAttestation> =
+        Vec::with_capacity(attester_slashings.len() * 2);
+    for attester_slashing in attester_slashings {
+        slashable_attestations.push(&attester_slashing.slashable_attestation_1);
+        slashable_attestations.push(&attester_slashing.slashable_attestation_2);
+    }
+
+    // Verify slashable attestations in parallel.
+    slashable_attestations
+        .par_iter()
+        .enumerate()
+        .try_for_each(|(i, slashable_attestation)| {
+            verify_slashable_attestation(&state, slashable_attestation, spec)
+                .map_err(|e| e.into_with_index(i))
+        })?;
+    let all_slashable_attestations_have_been_checked = true;
+
+    // Gather the slashable indices and preform the final verification and update the state in series.
     for (i, attester_slashing) in attester_slashings.iter().enumerate() {
-        let slashable_indices = verify_attester_slashing(&state, &attester_slashing, spec)
+        let should_verify_slashable_attestations = !all_slashable_attestations_have_been_checked;
+
+        verify_attester_slashing(
+            &state,
+            &attester_slashing,
+            should_verify_slashable_attestations,
+            spec,
+        )
+        .map_err(|e| e.into_with_index(i))?;
+
+        let slashable_indices = gather_attester_slashing_indices(&state, &attester_slashing)
             .map_err(|e| e.into_with_index(i))?;
+
         for i in slashable_indices {
             state.slash_validator(i as usize, spec)?;
         }
@@ -278,14 +322,20 @@ pub fn process_attestations(
         attestations.len() as u64 <= spec.max_attestations,
         Invalid::MaxAttestationsExceeded
     );
-    for (i, attestation) in attestations.iter().enumerate() {
-        // Build the previous epoch cache only if required by an attestation.
-        if attestation.data.slot.epoch(spec.slots_per_epoch) == state.previous_epoch(spec) {
-            state.build_epoch_cache(RelativeEpoch::Previous, spec)?;
-        }
 
-        validate_attestation(state, attestation, spec).map_err(|e| e.into_with_index(i))?;
+    // Ensure the previous epoch cache exists.
+    state.build_epoch_cache(RelativeEpoch::Previous, spec)?;
 
+    // Verify attestations in parallel.
+    attestations
+        .par_iter()
+        .enumerate()
+        .try_for_each(|(i, attestation)| {
+            validate_attestation(state, attestation, spec).map_err(|e| e.into_with_index(i))
+        })?;
+
+    // Update the state in series.
+    for attestation in attestations {
         let pending_attestation = PendingAttestation {
             data: attestation.data.clone(),
             aggregation_bitfield: attestation.aggregation_bitfield.clone(),
@@ -313,24 +363,53 @@ pub fn process_deposits(
         deposits.len() as u64 <= spec.max_deposits,
         Invalid::MaxDepositsExceeded
     );
-    for (i, deposit) in deposits.iter().enumerate() {
-        verify_deposit(state, deposit, VERIFY_DEPOSIT_MERKLE_PROOFS, spec)
-            .map_err(|e| e.into_with_index(i))?;
 
-        state
-            .process_deposit(
-                deposit.deposit_data.deposit_input.pubkey.clone(),
-                deposit.deposit_data.amount,
-                deposit
-                    .deposit_data
-                    .deposit_input
-                    .proof_of_possession
-                    .clone(),
-                deposit.deposit_data.deposit_input.withdrawal_credentials,
-                None,
-                spec,
-            )
-            .map_err(|_| Error::Invalid(Invalid::DepositProcessingFailed(i)))?;
+    // Verify deposits in parallel.
+    deposits
+        .par_iter()
+        .enumerate()
+        .try_for_each(|(i, deposit)| {
+            verify_deposit(state, deposit, VERIFY_DEPOSIT_MERKLE_PROOFS, spec)
+                .map_err(|e| e.into_with_index(i))
+        })?;
+
+    let public_key_to_index_hashmap = build_public_key_hashmap(&state);
+
+    // Check `state.deposit_index` and update the state in series.
+    for (i, deposit) in deposits.iter().enumerate() {
+        verify_deposit_index(state, deposit).map_err(|e| e.into_with_index(i))?;
+
+        // Get an `Option<u64>` where `u64` is the validator index if this deposit public key
+        // already exists in the beacon_state.
+        //
+        // This function also verifies the withdrawal credentials.
+        let validator_index =
+            get_existing_validator_index(state, deposit, &public_key_to_index_hashmap)
+                .map_err(|e| e.into_with_index(i))?;
+
+        let deposit_data = &deposit.deposit_data;
+        let deposit_input = &deposit.deposit_data.deposit_input;
+
+        if let Some(index) = validator_index {
+            // Update the existing validator balance.
+            safe_add_assign!(
+                state.validator_balances[index as usize],
+                deposit_data.amount
+            );
+        } else {
+            // Create a new validator.
+            let validator = Validator {
+                pubkey: deposit_input.pubkey.clone(),
+                withdrawal_credentials: deposit_input.withdrawal_credentials.clone(),
+                activation_epoch: spec.far_future_epoch,
+                exit_epoch: spec.far_future_epoch,
+                withdrawable_epoch: spec.far_future_epoch,
+                initiated_exit: false,
+                slashed: false,
+            };
+            state.validator_registry.push(validator);
+            state.validator_balances.push(deposit_data.amount);
+        }
 
         state.deposit_index += 1;
     }
@@ -353,9 +432,17 @@ pub fn process_exits(
         voluntary_exits.len() as u64 <= spec.max_voluntary_exits,
         Invalid::MaxExitsExceeded
     );
-    for (i, exit) in voluntary_exits.iter().enumerate() {
-        verify_exit(&state, exit, spec).map_err(|e| e.into_with_index(i))?;
 
+    // Verify exits in parallel.
+    voluntary_exits
+        .par_iter()
+        .enumerate()
+        .try_for_each(|(i, exit)| {
+            verify_exit(&state, exit, spec).map_err(|e| e.into_with_index(i))
+        })?;
+
+    // Update the state in series.
+    for exit in voluntary_exits {
         state.initiate_validator_exit(exit.validator_index as usize);
     }
 
@@ -377,8 +464,15 @@ pub fn process_transfers(
         transfers.len() as u64 <= spec.max_transfers,
         Invalid::MaxTransfersExceed
     );
+
+    transfers
+        .par_iter()
+        .enumerate()
+        .try_for_each(|(i, transfer)| {
+            verify_transfer(&state, transfer, spec).map_err(|e| e.into_with_index(i))
+        })?;
+
     for (i, transfer) in transfers.iter().enumerate() {
-        verify_transfer(&state, transfer, spec).map_err(|e| e.into_with_index(i))?;
         execute_transfer(state, transfer, spec).map_err(|e| e.into_with_index(i))?;
     }
 
diff --git a/eth2/state_processing/src/per_block_processing/errors.rs b/eth2/state_processing/src/per_block_processing/errors.rs
index b97d8bacc..a3e3ebad1 100644
--- a/eth2/state_processing/src/per_block_processing/errors.rs
+++ b/eth2/state_processing/src/per_block_processing/errors.rs
@@ -76,6 +76,10 @@ pub enum BlockInvalid {
     MaxExitsExceeded,
     MaxTransfersExceed,
     AttestationInvalid(usize, AttestationInvalid),
+    /// A `SlashableAttestation` inside an `AttesterSlashing` was invalid.
+    ///
+    /// To determine the offending `AttesterSlashing` index, divide the error message `usize` by two.
+    SlashableAttestationInvalid(usize, SlashableAttestationInvalid),
     AttesterSlashingInvalid(usize, AttesterSlashingInvalid),
     ProposerSlashingInvalid(usize, ProposerSlashingInvalid),
     DepositInvalid(usize, DepositInvalid),
@@ -147,6 +151,8 @@ pub enum AttestationInvalid {
     ///
     /// (attestation_data_shard, attestation_data_slot)
     NoCommitteeForShard(u64, Slot),
+    /// The validator index was unknown.
+    UnknownValidator(u64),
     /// The attestation signature verification failed.
     BadSignature,
     /// The shard block root was not set to zero. This is a phase 0 requirement.
@@ -233,6 +239,11 @@ impl Into<SlashableAttestationInvalid> for SlashableAttestationValidationError {
     }
 }
 
+impl_into_with_index_without_beacon_error!(
+    SlashableAttestationValidationError,
+    SlashableAttestationInvalid
+);
+
 /*
  * `ProposerSlashing` Validation
  */
@@ -292,6 +303,11 @@ pub enum DepositInvalid {
     ///
     /// (state_index, deposit_index)
     BadIndex(u64, u64),
+    /// The proof-of-possession does not match the given pubkey.
+    BadProofOfPossession,
+    /// The withdrawal credentials for the depositing validator did not match the withdrawal
+    /// credentials of an existing validator with the same public key.
+    BadWithdrawalCredentials,
     /// The specified `branch` and `index` did not form a valid proof that the deposit is included
     /// in the eth1 deposit root.
     BadMerkleProof,
diff --git a/eth2/state_processing/src/per_block_processing/validate_attestation.rs b/eth2/state_processing/src/per_block_processing/validate_attestation.rs
index 54bd2d332..b15360850 100644
--- a/eth2/state_processing/src/per_block_processing/validate_attestation.rs
+++ b/eth2/state_processing/src/per_block_processing/validate_attestation.rs
@@ -159,18 +159,16 @@ fn validate_attestation_signature_optional(
 
     if verify_signature {
         let attestation_epoch = attestation.data.slot.epoch(spec.slots_per_epoch);
-        verify!(
-            verify_attestation_signature(
-                state,
-                committee,
-                attestation_epoch,
-                &attestation.custody_bitfield,
-                &attestation.data,
-                &attestation.aggregate_signature,
-                spec
-            ),
-            Invalid::BadSignature
-        );
+        verify_attestation_signature(
+            state,
+            committee,
+            attestation_epoch,
+            &attestation.aggregation_bitfield,
+            &attestation.custody_bitfield,
+            &attestation.data,
+            &attestation.aggregate_signature,
+            spec,
+        )?;
     }
 
     // [TO BE REMOVED IN PHASE 1] Verify that `attestation.data.crosslink_data_root == ZERO_HASH`.
@@ -195,30 +193,45 @@ fn verify_attestation_signature(
     state: &BeaconState,
     committee: &[usize],
     attestation_epoch: Epoch,
+    aggregation_bitfield: &Bitfield,
     custody_bitfield: &Bitfield,
     attestation_data: &AttestationData,
     aggregate_signature: &AggregateSignature,
     spec: &ChainSpec,
-) -> bool {
+) -> Result<(), Error> {
     let mut aggregate_pubs = vec![AggregatePublicKey::new(); 2];
     let mut message_exists = vec![false; 2];
 
     for (i, v) in committee.iter().enumerate() {
-        let custody_bit = match custody_bitfield.get(i) {
-            Ok(bit) => bit,
-            // Invalidate signature if custody_bitfield.len() < committee
-            Err(_) => return false,
-        };
+        let validator_signed = aggregation_bitfield.get(i).map_err(|_| {
+            Error::Invalid(Invalid::BadAggregationBitfieldLength(
+                committee.len(),
+                aggregation_bitfield.len(),
+            ))
+        })?;
 
-        message_exists[custody_bit as usize] = true;
+        if validator_signed {
+            let custody_bit: bool = match custody_bitfield.get(i) {
+                Ok(bit) => bit,
+                // Invalidate signature if custody_bitfield.len() < committee
+                Err(_) => {
+                    return Err(Error::Invalid(Invalid::BadCustodyBitfieldLength(
+                        committee.len(),
+                        custody_bitfield.len(),
+                    )));
+                }
+            };
 
-        match state.validator_registry.get(*v as usize) {
-            Some(validator) => {
-                aggregate_pubs[custody_bit as usize].add(&validator.pubkey);
-            }
-            // Invalidate signature if validator index is unknown.
-            None => return false,
-        };
+            message_exists[custody_bit as usize] = true;
+
+            match state.validator_registry.get(*v as usize) {
+                Some(validator) => {
+                    aggregate_pubs[custody_bit as usize].add(&validator.pubkey);
+                }
+                // Return error if validator index is unknown.
+                None => return Err(Error::BeaconStateError(BeaconStateError::UnknownValidator)),
+            };
+        }
     }
 
     // Message when custody bitfield is `false`
@@ -251,5 +264,10 @@ fn verify_attestation_signature(
 
     let domain = spec.get_domain(attestation_epoch, Domain::Attestation, &state.fork);
 
-    aggregate_signature.verify_multiple(&messages[..], domain, &keys[..])
+    verify!(
+        aggregate_signature.verify_multiple(&messages[..], domain, &keys[..]),
+        Invalid::BadSignature
+    );
+
+    Ok(())
 }
diff --git a/eth2/state_processing/src/per_block_processing/verify_attester_slashing.rs b/eth2/state_processing/src/per_block_processing/verify_attester_slashing.rs
index 71ac97469..d126849b6 100644
--- a/eth2/state_processing/src/per_block_processing/verify_attester_slashing.rs
+++ b/eth2/state_processing/src/per_block_processing/verify_attester_slashing.rs
@@ -11,8 +11,9 @@ use types::*;
 pub fn verify_attester_slashing(
     state: &BeaconState,
     attester_slashing: &AttesterSlashing,
+    should_verify_slashable_attestations: bool,
     spec: &ChainSpec,
-) -> Result<Vec<u64>, Error> {
+) -> Result<(), Error> {
     let slashable_attestation_1 = &attester_slashing.slashable_attestation_1;
     let slashable_attestation_2 = &attester_slashing.slashable_attestation_2;
 
@@ -26,10 +27,27 @@ pub fn verify_attester_slashing(
         Invalid::NotSlashable
     );
 
-    verify_slashable_attestation(state, &slashable_attestation_1, spec)
-        .map_err(|e| Error::Invalid(Invalid::SlashableAttestation1Invalid(e.into())))?;
-    verify_slashable_attestation(state, &slashable_attestation_2, spec)
-        .map_err(|e| Error::Invalid(Invalid::SlashableAttestation2Invalid(e.into())))?;
+    if should_verify_slashable_attestations {
+        verify_slashable_attestation(state, &slashable_attestation_1, spec)
+            .map_err(|e| Error::Invalid(Invalid::SlashableAttestation1Invalid(e.into())))?;
+        verify_slashable_attestation(state, &slashable_attestation_2, spec)
+            .map_err(|e| Error::Invalid(Invalid::SlashableAttestation2Invalid(e.into())))?;
+    }
+
+    Ok(())
+}
+
+/// For a given attester slashing, return the indices able to be slashed.
+///
+/// Returns Ok(indices) if `indices.len() > 0`.
+///
+/// Spec v0.4.0
+pub fn gather_attester_slashing_indices(
+    state: &BeaconState,
+    attester_slashing: &AttesterSlashing,
+) -> Result<Vec<u64>, Error> {
+    let slashable_attestation_1 = &attester_slashing.slashable_attestation_1;
+    let slashable_attestation_2 = &attester_slashing.slashable_attestation_2;
 
     let mut slashable_indices = vec![];
     for i in &slashable_attestation_1.validator_indices {
@@ -38,7 +56,7 @@ pub fn verify_attester_slashing(
             .get(*i as usize)
             .ok_or_else(|| Error::Invalid(Invalid::UnknownValidator(*i)))?;
 
-        if slashable_attestation_1.validator_indices.contains(&i) & !validator.slashed {
+        if slashable_attestation_2.validator_indices.contains(&i) & !validator.slashed {
             slashable_indices.push(*i);
         }
     }
diff --git a/eth2/state_processing/src/per_block_processing/verify_deposit.rs b/eth2/state_processing/src/per_block_processing/verify_deposit.rs
index 69dae1533..1aabbb973 100644
--- a/eth2/state_processing/src/per_block_processing/verify_deposit.rs
+++ b/eth2/state_processing/src/per_block_processing/verify_deposit.rs
@@ -3,13 +3,19 @@ use hashing::hash;
 use merkle_proof::verify_merkle_proof;
 use ssz::ssz_encode;
 use ssz_derive::Encode;
+use std::collections::HashMap;
 use types::*;
 
+pub type PublicKeyValidatorIndexHashmap = HashMap<PublicKey, u64>;
+
 /// Indicates if a `Deposit` is valid to be included in a block in the current epoch of the given
 /// state.
 ///
 /// Returns `Ok(())` if the `Deposit` is valid, otherwise indicates the reason for invalidity.
 ///
+/// This function _does not_ check `state.deposit_index` so this function may be run in parallel.
+/// See the `verify_deposit_index` function for this.
+///
 /// Note: this function is incomplete.
 ///
 /// Spec v0.4.0
@@ -20,8 +26,15 @@ pub fn verify_deposit(
     spec: &ChainSpec,
 ) -> Result<(), Error> {
     verify!(
-        deposit.index == state.deposit_index,
-        Invalid::BadIndex(state.deposit_index, deposit.index)
+        deposit
+            .deposit_data
+            .deposit_input
+            .validate_proof_of_possession(
+                state.slot.epoch(spec.slots_per_epoch),
+                &state.fork,
+                spec
+            ),
+        Invalid::BadProofOfPossession
     );
 
     if verify_merkle_branch {
@@ -34,6 +47,50 @@ pub fn verify_deposit(
     Ok(())
 }
 
+/// Verify that the `Deposit` index is correct.
+///
+/// Spec v0.4.0
+pub fn verify_deposit_index(state: &BeaconState, deposit: &Deposit) -> Result<(), Error> {
+    verify!(
+        deposit.index == state.deposit_index,
+        Invalid::BadIndex(state.deposit_index, deposit.index)
+    );
+
+    Ok(())
+}
+
+pub fn build_public_key_hashmap(state: &BeaconState) -> PublicKeyValidatorIndexHashmap {
+    let mut hashmap = HashMap::with_capacity(state.validator_registry.len());
+
+    for (i, validator) in state.validator_registry.iter().enumerate() {
+        hashmap.insert(validator.pubkey.clone(), i as u64);
+    }
+
+    hashmap
+}
+
+pub fn get_existing_validator_index(
+    state: &BeaconState,
+    deposit: &Deposit,
+    pubkey_map: &HashMap<PublicKey, u64>,
+) -> Result<Option<u64>, Error> {
+    let deposit_input = &deposit.deposit_data.deposit_input;
+
+    let validator_index = pubkey_map.get(&deposit_input.pubkey).and_then(|i| Some(*i));
+
+    match validator_index {
+        None => Ok(None),
+        Some(index) => {
+            verify!(
+                deposit_input.withdrawal_credentials
+                    == state.validator_registry[index as usize].withdrawal_credentials,
+                Invalid::BadWithdrawalCredentials
+            );
+            Ok(Some(index))
+        }
+    }
+}
+
 /// Verify that a deposit is included in the state's eth1 deposit root.
 ///
 /// Spec v0.4.0
diff --git a/eth2/state_processing/src/per_epoch_processing.rs b/eth2/state_processing/src/per_epoch_processing.rs
index bd8aca3c4..4abbe012c 100644
--- a/eth2/state_processing/src/per_epoch_processing.rs
+++ b/eth2/state_processing/src/per_epoch_processing.rs
@@ -1,11 +1,11 @@
 use attester_sets::AttesterSets;
 use errors::EpochProcessingError as Error;
-use inclusion_distance::{inclusion_distance, inclusion_slot};
+use fnv::FnvHashMap;
+use fnv::FnvHashSet;
 use integer_sqrt::IntegerSquareRoot;
-use log::debug;
 use rayon::prelude::*;
 use ssz::TreeHash;
-use std::collections::{HashMap, HashSet};
+use std::collections::HashMap;
 use std::iter::FromIterator;
 use types::{validator_registry::get_active_validator_indices, *};
 use winning_root::{winning_root, WinningRoot};
@@ -16,29 +16,31 @@ pub mod inclusion_distance;
 pub mod tests;
 pub mod winning_root;
 
-pub fn per_epoch_processing(state: &mut BeaconState, spec: &ChainSpec) -> Result<(), Error> {
-    let current_epoch = state.current_epoch(spec);
-    let previous_epoch = state.previous_epoch(spec);
-    let next_epoch = state.next_epoch(spec);
+/// Maps a shard to a winning root.
+///
+/// It is generated during crosslink processing and later used to reward/penalize validators.
+pub type WinningRootHashSet = HashMap<u64, WinningRoot>;
 
-    debug!(
-        "Starting per-epoch processing on epoch {}...",
-        state.current_epoch(spec)
-    );
+/// Performs per-epoch processing on some BeaconState.
+///
+/// Mutates the given `BeaconState`, returning early if an error is encountered. If an error is
+/// returned, a state might be "half-processed" and therefore in an invalid state.
+///
+/// Spec v0.4.0
+pub fn per_epoch_processing(state: &mut BeaconState, spec: &ChainSpec) -> Result<(), Error> {
+    let previous_epoch = state.previous_epoch(spec);
 
     // Ensure all of the caches are built.
     state.build_epoch_cache(RelativeEpoch::Previous, spec)?;
     state.build_epoch_cache(RelativeEpoch::Current, spec)?;
     state.build_epoch_cache(RelativeEpoch::Next, spec)?;
 
-    let attesters = AttesterSets::new(&state, spec)?;
+    let attesters = calculate_attester_sets(&state, spec)?;
 
-    let active_validator_indices = get_active_validator_indices(
-        &state.validator_registry,
-        state.slot.epoch(spec.slots_per_epoch),
-    );
+    let active_validator_indices = calculate_active_validator_indices(&state, spec);
 
     let current_total_balance = state.get_total_balance(&active_validator_indices[..], spec);
+
     let previous_total_balance = state.get_total_balance(
         &get_active_validator_indices(&state.validator_registry, previous_epoch)[..],
         spec,
@@ -59,11 +61,9 @@ pub fn per_epoch_processing(state: &mut BeaconState, spec: &ChainSpec) -> Result
     let winning_root_for_shards = process_crosslinks(state, spec)?;
 
     // Rewards and Penalities
-    let active_validator_indices_hashset: HashSet<usize> =
-        HashSet::from_iter(active_validator_indices.iter().cloned());
     process_rewards_and_penalities(
         state,
-        active_validator_indices_hashset,
+        &active_validator_indices,
         &attesters,
         previous_total_balance,
         &winning_root_for_shards,
@@ -77,38 +77,43 @@ pub fn per_epoch_processing(state: &mut BeaconState, spec: &ChainSpec) -> Result
     process_validator_registry(state, spec)?;
 
     // Final updates
-    let active_tree_root = get_active_validator_indices(
-        &state.validator_registry,
-        next_epoch + Epoch::from(spec.activation_exit_delay),
-    )
-    .hash_tree_root();
-    state.latest_active_index_roots[(next_epoch.as_usize()
-        + spec.activation_exit_delay as usize)
-        % spec.latest_active_index_roots_length] = Hash256::from_slice(&active_tree_root[..]);
-
-    state.latest_slashed_balances[next_epoch.as_usize() % spec.latest_slashed_exit_length] =
-        state.latest_slashed_balances[current_epoch.as_usize() % spec.latest_slashed_exit_length];
-    state.latest_randao_mixes[next_epoch.as_usize() % spec.latest_randao_mixes_length] = state
-        .get_randao_mix(current_epoch, spec)
-        .and_then(|x| Some(*x))
-        .ok_or_else(|| Error::NoRandaoSeed)?;
-    state.latest_attestations = state
-        .latest_attestations
-        .iter()
-        .filter(|a| a.data.slot.epoch(spec.slots_per_epoch) >= current_epoch)
-        .cloned()
-        .collect();
+    update_active_tree_index_roots(state, spec)?;
+    update_latest_slashed_balances(state, spec);
+    clean_attestations(state, spec);
 
     // Rotate the epoch caches to suit the epoch transition.
     state.advance_caches();
 
-    debug!("Epoch transition complete.");
-
     Ok(())
 }
 
+/// Returns a list of active validator indices for the state's current epoch.
+///
 /// Spec v0.4.0
-fn process_eth1_data(state: &mut BeaconState, spec: &ChainSpec) {
+pub fn calculate_active_validator_indices(state: &BeaconState, spec: &ChainSpec) -> Vec<usize> {
+    get_active_validator_indices(
+        &state.validator_registry,
+        state.slot.epoch(spec.slots_per_epoch),
+    )
+}
+
+/// Calculates various sets of attesters, including:
+///
+/// - current epoch attesters
+/// - current epoch boundary attesters
+/// - previous epoch attesters
+/// - etc.
+///
+/// Spec v0.4.0
+pub fn calculate_attester_sets(
+    state: &BeaconState,
+    spec: &ChainSpec,
+) -> Result<AttesterSets, BeaconStateError> {
+    AttesterSets::new(&state, spec)
+}
+
+/// Spec v0.4.0
+pub fn process_eth1_data(state: &mut BeaconState, spec: &ChainSpec) {
     let next_epoch = state.next_epoch(spec);
     let voting_period = spec.epochs_per_eth1_voting_period;
 
@@ -122,8 +127,15 @@ fn process_eth1_data(state: &mut BeaconState, spec: &ChainSpec) {
     }
 }
 
+/// Update the following fields on the `BeaconState`:
+///
+/// - `justification_bitfield`.
+/// - `finalized_epoch`
+/// - `justified_epoch`
+/// - `previous_justified_epoch`
+///
 /// Spec v0.4.0
-fn process_justification(
+pub fn process_justification(
     state: &mut BeaconState,
     current_total_balance: u64,
     previous_total_balance: u64,
@@ -199,9 +211,14 @@ fn process_justification(
     state.justified_epoch = new_justified_epoch;
 }
 
-pub type WinningRootHashSet = HashMap<u64, WinningRoot>;
-
-fn process_crosslinks(
+/// Updates the following fields on the `BeaconState`:
+///
+/// - `latest_crosslinks`
+///
+/// Also returns a `WinningRootHashSet` for later use during epoch processing.
+///
+/// Spec v0.4.0
+pub fn process_crosslinks(
     state: &mut BeaconState,
     spec: &ChainSpec,
 ) -> Result<WinningRootHashSet, Error> {
@@ -259,10 +276,14 @@ fn process_crosslinks(
     Ok(winning_root_for_shards)
 }
 
+/// Updates the following fields on the BeaconState:
+///
+/// - `validator_balances`
+///
 /// Spec v0.4.0
-fn process_rewards_and_penalities(
+pub fn process_rewards_and_penalities(
     state: &mut BeaconState,
-    active_validator_indices: HashSet<usize>,
+    active_validator_indices: &[usize],
     attesters: &AttesterSets,
     previous_total_balance: u64,
     winning_root_for_shards: &WinningRootHashSet,
@@ -270,6 +291,9 @@ fn process_rewards_and_penalities(
 ) -> Result<(), Error> {
     let next_epoch = state.next_epoch(spec);
 
+    let active_validator_indices: FnvHashSet<usize> =
+        FnvHashSet::from_iter(active_validator_indices.iter().cloned());
+
     let previous_epoch_attestations: Vec<&PendingAttestation> = state
         .latest_attestations
         .par_iter()
@@ -281,110 +305,161 @@ fn process_rewards_and_penalities(
     if base_reward_quotient == 0 {
         return Err(Error::BaseRewardQuotientIsZero);
     }
+    if previous_total_balance == 0 {
+        return Err(Error::PreviousTotalBalanceIsZero);
+    }
+
+    // Map is ValidatorIndex -> ProposerIndex
+    let mut inclusion_slots: FnvHashMap<usize, (Slot, usize)> = FnvHashMap::default();
+    for a in &previous_epoch_attestations {
+        let participants =
+            state.get_attestation_participants(&a.data, &a.aggregation_bitfield, spec)?;
+        let inclusion_distance = (a.inclusion_slot - a.data.slot).as_u64();
+        for participant in participants {
+            if let Some((existing_distance, _)) = inclusion_slots.get(&participant) {
+                if *existing_distance <= inclusion_distance {
+                    continue;
+                }
+            }
+            let proposer_index = state
+                .get_beacon_proposer_index(a.data.slot, spec)
+                .map_err(|_| Error::UnableToDetermineProducer)?;
+            inclusion_slots.insert(
+                participant,
+                (Slot::from(inclusion_distance), proposer_index),
+            );
+        }
+    }
 
     // Justification and finalization
 
     let epochs_since_finality = next_epoch - state.finalized_epoch;
 
     if epochs_since_finality <= 4 {
-        for index in 0..state.validator_balances.len() {
-            let base_reward = state.base_reward(index, base_reward_quotient, spec);
+        state.validator_balances = state
+            .validator_balances
+            .par_iter()
+            .enumerate()
+            .map(|(index, &balance)| {
+                let mut balance = balance;
+                let base_reward = state.base_reward(index, base_reward_quotient, spec);
 
-            // Expected FFG source
-            if attesters.previous_epoch.indices.contains(&index) {
-                safe_add_assign!(
-                    state.validator_balances[index],
-                    base_reward * attesters.previous_epoch.balance / previous_total_balance
-                );
-            } else if active_validator_indices.contains(&index) {
-                safe_sub_assign!(state.validator_balances[index], base_reward);
-            }
-
-            // Expected FFG target
-            if attesters.previous_epoch_boundary.indices.contains(&index) {
-                safe_add_assign!(
-                    state.validator_balances[index],
-                    base_reward * attesters.previous_epoch_boundary.balance
-                        / previous_total_balance
-                );
-            } else if active_validator_indices.contains(&index) {
-                safe_sub_assign!(state.validator_balances[index], base_reward);
-            }
-
-            // Expected beacon chain head
-            if attesters.previous_epoch_head.indices.contains(&index) {
-                safe_add_assign!(
-                    state.validator_balances[index],
-                    base_reward * attesters.previous_epoch_head.balance / previous_total_balance
-                );
-            } else if active_validator_indices.contains(&index) {
-                safe_sub_assign!(state.validator_balances[index], base_reward);
-            }
-        }
-
-        // Inclusion distance
-        for &index in &attesters.previous_epoch.indices {
-            let base_reward = state.base_reward(index, base_reward_quotient, spec);
-            let inclusion_distance =
-                inclusion_distance(state, &previous_epoch_attestations, index, spec)?;
-
-            safe_add_assign!(
-                state.validator_balances[index],
-                base_reward * spec.min_attestation_inclusion_delay / inclusion_distance
-            )
-        }
-    } else {
-        for index in 0..state.validator_balances.len() {
-            let inactivity_penalty =
-                state.inactivity_penalty(index, epochs_since_finality, base_reward_quotient, spec);
-
-            if active_validator_indices.contains(&index) {
-                if !attesters.previous_epoch.indices.contains(&index) {
-                    safe_sub_assign!(state.validator_balances[index], inactivity_penalty);
-                }
-                if !attesters.previous_epoch_boundary.indices.contains(&index) {
-                    safe_sub_assign!(state.validator_balances[index], inactivity_penalty);
-                }
-                if !attesters.previous_epoch_head.indices.contains(&index) {
-                    safe_sub_assign!(state.validator_balances[index], inactivity_penalty);
-                }
-
-                if state.validator_registry[index].slashed {
-                    let base_reward = state.base_reward(index, base_reward_quotient, spec);
-                    safe_sub_assign!(
-                        state.validator_balances[index],
-                        2 * inactivity_penalty + base_reward
+                // Expected FFG source
+                if attesters.previous_epoch.indices.contains(&index) {
+                    safe_add_assign!(
+                        balance,
+                        base_reward * attesters.previous_epoch.balance / previous_total_balance
                     );
+                } else if active_validator_indices.contains(&index) {
+                    safe_sub_assign!(balance, base_reward);
                 }
-            }
-        }
 
-        for &index in &attesters.previous_epoch.indices {
-            let base_reward = state.base_reward(index, base_reward_quotient, spec);
-            let inclusion_distance =
-                inclusion_distance(state, &previous_epoch_attestations, index, spec)?;
+                // Expected FFG target
+                if attesters.previous_epoch_boundary.indices.contains(&index) {
+                    safe_add_assign!(
+                        balance,
+                        base_reward * attesters.previous_epoch_boundary.balance
+                            / previous_total_balance
+                    );
+                } else if active_validator_indices.contains(&index) {
+                    safe_sub_assign!(balance, base_reward);
+                }
 
-            safe_sub_assign!(
-                state.validator_balances[index],
-                base_reward
-                    - base_reward * spec.min_attestation_inclusion_delay / inclusion_distance
-            );
-        }
+                // Expected beacon chain head
+                if attesters.previous_epoch_head.indices.contains(&index) {
+                    safe_add_assign!(
+                        balance,
+                        base_reward * attesters.previous_epoch_head.balance
+                            / previous_total_balance
+                    );
+                } else if active_validator_indices.contains(&index) {
+                    safe_sub_assign!(balance, base_reward);
+                };
+
+                if attesters.previous_epoch.indices.contains(&index) {
+                    let base_reward = state.base_reward(index, base_reward_quotient, spec);
+
+                    let (inclusion_distance, _) = inclusion_slots
+                        .get(&index)
+                        .expect("Inconsistent inclusion_slots.");
+
+                    if *inclusion_distance > 0 {
+                        safe_add_assign!(
+                            balance,
+                            base_reward * spec.min_attestation_inclusion_delay
+                                / inclusion_distance.as_u64()
+                        )
+                    }
+                }
+
+                balance
+            })
+            .collect();
+    } else {
+        state.validator_balances = state
+            .validator_balances
+            .par_iter()
+            .enumerate()
+            .map(|(index, &balance)| {
+                let mut balance = balance;
+
+                let inactivity_penalty = state.inactivity_penalty(
+                    index,
+                    epochs_since_finality,
+                    base_reward_quotient,
+                    spec,
+                );
+
+                if active_validator_indices.contains(&index) {
+                    if !attesters.previous_epoch.indices.contains(&index) {
+                        safe_sub_assign!(balance, inactivity_penalty);
+                    }
+                    if !attesters.previous_epoch_boundary.indices.contains(&index) {
+                        safe_sub_assign!(balance, inactivity_penalty);
+                    }
+                    if !attesters.previous_epoch_head.indices.contains(&index) {
+                        safe_sub_assign!(balance, inactivity_penalty);
+                    }
+
+                    if state.validator_registry[index].slashed {
+                        let base_reward = state.base_reward(index, base_reward_quotient, spec);
+                        safe_sub_assign!(balance, 2 * inactivity_penalty + base_reward);
+                    }
+                }
+
+                if attesters.previous_epoch.indices.contains(&index) {
+                    let base_reward = state.base_reward(index, base_reward_quotient, spec);
+
+                    let (inclusion_distance, _) = inclusion_slots
+                        .get(&index)
+                        .expect("Inconsistent inclusion_slots.");
+
+                    if *inclusion_distance > 0 {
+                        safe_add_assign!(
+                            balance,
+                            base_reward * spec.min_attestation_inclusion_delay
+                                / inclusion_distance.as_u64()
+                        )
+                    }
+                }
+
+                balance
+            })
+            .collect();
     }
 
     // Attestation inclusion
+    //
 
     for &index in &attesters.previous_epoch.indices {
-        let inclusion_slot = inclusion_slot(state, &previous_epoch_attestations[..], index, spec)?;
+        let (_, proposer_index) = inclusion_slots
+            .get(&index)
+            .ok_or_else(|| Error::InclusionSlotsInconsistent(index))?;
 
-        let proposer_index = state
-            .get_beacon_proposer_index(inclusion_slot, spec)
-            .map_err(|_| Error::UnableToDetermineProducer)?;
-
-        let base_reward = state.base_reward(proposer_index, base_reward_quotient, spec);
+        let base_reward = state.base_reward(*proposer_index, base_reward_quotient, spec);
 
         safe_add_assign!(
-            state.validator_balances[proposer_index],
+            state.validator_balances[*proposer_index],
             base_reward / spec.attestation_inclusion_reward_quotient
         );
     }
@@ -413,8 +488,8 @@ fn process_rewards_and_penalities(
             if let Some(winning_root) = winning_root_for_shards.get(&shard) {
                 // Hash set de-dedups and (hopefully) offers a speed improvement from faster
                 // lookups.
-                let attesting_validator_indices: HashSet<usize> =
-                    HashSet::from_iter(winning_root.attesting_validator_indices.iter().cloned());
+                let attesting_validator_indices: FnvHashSet<usize> =
+                    FnvHashSet::from_iter(winning_root.attesting_validator_indices.iter().cloned());
 
                 for &index in &crosslink_committee {
                     let base_reward = state.base_reward(index, base_reward_quotient, spec);
@@ -443,8 +518,10 @@ fn process_rewards_and_penalities(
     Ok(())
 }
 
-// Spec v0.4.0
-fn process_validator_registry(state: &mut BeaconState, spec: &ChainSpec) -> Result<(), Error> {
+/// Peforms a validator registry update, if required.
+///
+/// Spec v0.4.0
+pub fn process_validator_registry(state: &mut BeaconState, spec: &ChainSpec) -> Result<(), Error> {
     let current_epoch = state.current_epoch(spec);
     let next_epoch = state.next_epoch(spec);
 
@@ -489,3 +566,51 @@ fn process_validator_registry(state: &mut BeaconState, spec: &ChainSpec) -> Resu
 
     Ok(())
 }
+
+/// Updates the state's `latest_active_index_roots` field with a tree hash the active validator
+/// indices for the next epoch.
+///
+/// Spec v0.4.0
+pub fn update_active_tree_index_roots(
+    state: &mut BeaconState,
+    spec: &ChainSpec,
+) -> Result<(), Error> {
+    let next_epoch = state.next_epoch(spec);
+
+    let active_tree_root = get_active_validator_indices(
+        &state.validator_registry,
+        next_epoch + Epoch::from(spec.activation_exit_delay),
+    )
+    .hash_tree_root();
+
+    state.latest_active_index_roots[(next_epoch.as_usize()
+        + spec.activation_exit_delay as usize)
+        % spec.latest_active_index_roots_length] = Hash256::from_slice(&active_tree_root[..]);
+
+    Ok(())
+}
+
+/// Advances the state's `latest_slashed_balances` field.
+///
+/// Spec v0.4.0
+pub fn update_latest_slashed_balances(state: &mut BeaconState, spec: &ChainSpec) {
+    let current_epoch = state.current_epoch(spec);
+    let next_epoch = state.next_epoch(spec);
+
+    state.latest_slashed_balances[next_epoch.as_usize() % spec.latest_slashed_exit_length] =
+        state.latest_slashed_balances[current_epoch.as_usize() % spec.latest_slashed_exit_length];
+}
+
+/// Removes all pending attestations from the previous epoch.
+///
+/// Spec v0.4.0
+pub fn clean_attestations(state: &mut BeaconState, spec: &ChainSpec) {
+    let current_epoch = state.current_epoch(spec);
+
+    state.latest_attestations = state
+        .latest_attestations
+        .iter()
+        .filter(|a| a.data.slot.epoch(spec.slots_per_epoch) >= current_epoch)
+        .cloned()
+        .collect();
+}
diff --git a/eth2/state_processing/src/per_epoch_processing/attester_sets.rs b/eth2/state_processing/src/per_epoch_processing/attester_sets.rs
index 2b674e1bc..03f49c1d3 100644
--- a/eth2/state_processing/src/per_epoch_processing/attester_sets.rs
+++ b/eth2/state_processing/src/per_epoch_processing/attester_sets.rs
@@ -1,31 +1,55 @@
-use std::collections::HashSet;
+use fnv::FnvHashSet;
 use types::*;
 
+/// A set of validator indices, along with the total balance of all those attesters.
 #[derive(Default)]
 pub struct Attesters {
-    pub indices: HashSet<usize>,
+    /// A set of validator indices.
+    pub indices: FnvHashSet<usize>,
+    /// The total balance of all validators in `self.indices`.
     pub balance: u64,
 }
 
 impl Attesters {
+    /// Add the given indices to the set, incrementing the sets balance by the provided balance.
     fn add(&mut self, additional_indices: &[usize], additional_balance: u64) {
         self.indices.reserve(additional_indices.len());
         for i in additional_indices {
             self.indices.insert(*i);
         }
-        self.balance.saturating_add(additional_balance);
+        self.balance = self.balance.saturating_add(additional_balance);
     }
 }
 
+/// A collection of `Attester` objects, representing set of attesters that are rewarded/penalized
+/// during an epoch transition.
 pub struct AttesterSets {
+    /// All validators who attested during the state's current epoch.
     pub current_epoch: Attesters,
+    /// All validators who attested that the beacon block root of the first slot of the state's
+    /// current epoch is the same as the one stored in this state.
+    ///
+    /// In short validators who agreed with the state about the first slot of the current epoch.
     pub current_epoch_boundary: Attesters,
+    /// All validators who attested during the state's previous epoch.
     pub previous_epoch: Attesters,
+    /// All validators who attested that the beacon block root of the first slot of the state's
+    /// previous epoch is the same as the one stored in this state.
+    ///
+    /// In short, validators who agreed with the state about the first slot of the previous epoch.
     pub previous_epoch_boundary: Attesters,
+    /// All validators who attested that the beacon block root at the pending attestation's slot is
+    /// the same as the one stored in this state.
+    ///
+    /// In short, validators who agreed with the state about the current beacon block root when
+    /// they attested.
     pub previous_epoch_head: Attesters,
 }
 
 impl AttesterSets {
+    /// Loop through all attestations in the state and instantiate a complete `AttesterSets` struct.
+    ///
+    /// Spec v0.4.0
     pub fn new(state: &BeaconState, spec: &ChainSpec) -> Result<Self, BeaconStateError> {
         let mut current_epoch = Attesters::default();
         let mut current_epoch_boundary = Attesters::default();
@@ -67,10 +91,17 @@ impl AttesterSets {
     }
 }
 
+/// Returns `true` if some `PendingAttestation` is from the supplied `epoch`.
+///
+/// Spec v0.4.0
 fn is_from_epoch(a: &PendingAttestation, epoch: Epoch, spec: &ChainSpec) -> bool {
     a.data.slot.epoch(spec.slots_per_epoch) == epoch
 }
 
+/// Returns `true` if a `PendingAttestation` and `BeaconState` share the same beacon block hash for
+/// the first slot of the given epoch.
+///
+/// Spec v0.4.0
 fn has_common_epoch_boundary_root(
     a: &PendingAttestation,
     state: &BeaconState,
@@ -85,6 +116,10 @@ fn has_common_epoch_boundary_root(
     Ok(a.data.epoch_boundary_root == state_boundary_root)
 }
 
+/// Returns `true` if a `PendingAttestation` and `BeaconState` share the same beacon block hash for
+/// the current slot of the `PendingAttestation`.
+///
+/// Spec v0.4.0
 fn has_common_beacon_block_root(
     a: &PendingAttestation,
     state: &BeaconState,
diff --git a/eth2/state_processing/src/per_epoch_processing/errors.rs b/eth2/state_processing/src/per_epoch_processing/errors.rs
index 51e9b253c..c60e00cae 100644
--- a/eth2/state_processing/src/per_epoch_processing/errors.rs
+++ b/eth2/state_processing/src/per_epoch_processing/errors.rs
@@ -6,6 +6,13 @@ pub enum EpochProcessingError {
     NoBlockRoots,
     BaseRewardQuotientIsZero,
     NoRandaoSeed,
+    PreviousTotalBalanceIsZero,
+    InclusionDistanceZero,
+    /// Unable to get the inclusion distance for a validator that should have an inclusion
+    /// distance. This indicates an internal inconsistency.
+    ///
+    /// (validator_index)
+    InclusionSlotsInconsistent(usize),
     BeaconStateError(BeaconStateError),
     InclusionError(InclusionError),
 }
diff --git a/eth2/state_processing/src/per_epoch_processing/tests.rs b/eth2/state_processing/src/per_epoch_processing/tests.rs
index 627df858b..69450edcd 100644
--- a/eth2/state_processing/src/per_epoch_processing/tests.rs
+++ b/eth2/state_processing/src/per_epoch_processing/tests.rs
@@ -1,21 +1,21 @@
 #![cfg(test)]
 use crate::per_epoch_processing;
 use env_logger::{Builder, Env};
-use types::beacon_state::BeaconStateBuilder;
+use types::test_utils::TestingBeaconStateBuilder;
 use types::*;
 
 #[test]
 fn runs_without_error() {
     Builder::from_env(Env::default().default_filter_or("error")).init();
 
-    let mut builder = BeaconStateBuilder::new(8);
-    builder.spec = ChainSpec::few_validators();
+    let spec = ChainSpec::few_validators();
 
-    builder.build().unwrap();
-    builder.teleport_to_end_of_epoch(builder.spec.genesis_epoch + 4);
+    let mut builder = TestingBeaconStateBuilder::from_deterministic_keypairs(8, &spec);
 
-    let mut state = builder.cloned_state();
+    let target_slot = (spec.genesis_epoch + 4).end_slot(spec.slots_per_epoch);
+    builder.teleport_to_slot(target_slot, &spec);
 
-    let spec = &builder.spec;
-    per_epoch_processing(&mut state, spec).unwrap();
+    let (mut state, _keypairs) = builder.build();
+
+    per_epoch_processing(&mut state, &spec).unwrap();
 }
diff --git a/eth2/types/Cargo.toml b/eth2/types/Cargo.toml
index ea1343dba..27aef19d6 100644
--- a/eth2/types/Cargo.toml
+++ b/eth2/types/Cargo.toml
@@ -7,6 +7,7 @@ edition = "2018"
 [dependencies]
 bls = { path = "../utils/bls" }
 boolean-bitfield = { path = "../utils/boolean-bitfield" }
+dirs = "1.0"
 ethereum-types = "0.5"
 hashing = { path = "../utils/hashing" }
 honey-badger-split =  { path = "../utils/honey-badger-split" }
@@ -17,6 +18,7 @@ rand = "0.5.5"
 serde = "1.0"
 serde_derive = "1.0"
 serde_json = "1.0"
+serde_yaml = "0.8"
 slog = "^2.2.3"
 ssz = { path = "../utils/ssz" }
 ssz_derive = { path = "../utils/ssz_derive" }
diff --git a/eth2/types/src/attester_slashing.rs b/eth2/types/src/attester_slashing.rs
index 1cb671960..7a0752b6a 100644
--- a/eth2/types/src/attester_slashing.rs
+++ b/eth2/types/src/attester_slashing.rs
@@ -4,10 +4,6 @@ use serde_derive::Serialize;
 use ssz_derive::{Decode, Encode, TreeHash};
 use test_random_derive::TestRandom;
 
-mod builder;
-
-pub use builder::AttesterSlashingBuilder;
-
 /// Two conflicting attestations.
 ///
 /// Spec v0.4.0
diff --git a/eth2/types/src/beacon_block.rs b/eth2/types/src/beacon_block.rs
index 2e1e24ef7..b67c866a4 100644
--- a/eth2/types/src/beacon_block.rs
+++ b/eth2/types/src/beacon_block.rs
@@ -1,9 +1,9 @@
 use crate::test_utils::TestRandom;
-use crate::{BeaconBlockBody, ChainSpec, Eth1Data, Hash256, Slot};
+use crate::{BeaconBlockBody, ChainSpec, Eth1Data, Hash256, Proposal, Slot};
 use bls::Signature;
 use rand::RngCore;
 use serde_derive::Serialize;
-use ssz::TreeHash;
+use ssz::{SignedRoot, TreeHash};
 use ssz_derive::{Decode, Encode, SignedRoot, TreeHash};
 use test_random_derive::TestRandom;
 
@@ -23,6 +23,8 @@ pub struct BeaconBlock {
 
 impl BeaconBlock {
     /// Produce the first block of the Beacon Chain.
+    ///
+    /// Spec v0.4.0
     pub fn genesis(state_root: Hash256, spec: &ChainSpec) -> BeaconBlock {
         BeaconBlock {
             slot: spec.genesis_slot,
@@ -33,7 +35,6 @@ impl BeaconBlock {
                 deposit_root: spec.zero_hash,
                 block_hash: spec.zero_hash,
             },
-            signature: spec.empty_signature.clone(),
             body: BeaconBlockBody {
                 proposer_slashings: vec![],
                 attester_slashings: vec![],
@@ -42,13 +43,28 @@ impl BeaconBlock {
                 voluntary_exits: vec![],
                 transfers: vec![],
             },
+            signature: spec.empty_signature.clone(),
         }
     }
 
     /// Returns the `hash_tree_root` of the block.
+    ///
+    /// Spec v0.4.0
     pub fn canonical_root(&self) -> Hash256 {
         Hash256::from_slice(&self.hash_tree_root()[..])
     }
+
+    /// Returns an unsigned proposal for block.
+    ///
+    /// Spec v0.4.0
+    pub fn proposal(&self, spec: &ChainSpec) -> Proposal {
+        Proposal {
+            slot: self.slot,
+            shard: spec.beacon_chain_shard_number,
+            block_root: Hash256::from_slice(&self.signed_root()),
+            signature: spec.empty_signature.clone(),
+        }
+    }
 }
 
 #[cfg(test)]
diff --git a/eth2/types/src/beacon_state.rs b/eth2/types/src/beacon_state.rs
index 809408b32..b4faa6a49 100644
--- a/eth2/types/src/beacon_state.rs
+++ b/eth2/types/src/beacon_state.rs
@@ -1,15 +1,13 @@
 use self::epoch_cache::EpochCache;
 use crate::test_utils::TestRandom;
 use crate::{validator_registry::get_active_validator_indices, *};
-use bls::verify_proof_of_possession;
 use helpers::*;
 use honey_badger_split::SplitExt;
 use int_to_bytes::int_to_bytes32;
 use log::{debug, error, trace};
 use rand::RngCore;
-use rayon::prelude::*;
 use serde_derive::Serialize;
-use ssz::{hash, Decodable, DecodeError, Encodable, SszStream, TreeHash};
+use ssz::{hash, Decodable, DecodeError, Encodable, SignedRoot, SszStream, TreeHash};
 use std::collections::HashMap;
 use swap_or_not_shuffle::shuffle_list;
 
@@ -114,18 +112,18 @@ pub struct BeaconState {
 
 impl BeaconState {
     /// Produce the first state of the Beacon Chain.
-    pub fn genesis_without_validators(
-        genesis_time: u64,
-        latest_eth1_data: Eth1Data,
-        spec: &ChainSpec,
-    ) -> Result<BeaconState, Error> {
-        debug!("Creating genesis state (without validator processing).");
+    ///
+    /// This does not fully build a genesis beacon state, it omits processing of initial validator
+    /// deposits. To obtain a full genesis beacon state, use the `BeaconStateBuilder`.
+    ///
+    /// Spec v0.4.0
+    pub fn genesis(genesis_time: u64, latest_eth1_data: Eth1Data, spec: &ChainSpec) -> BeaconState {
         let initial_crosslink = Crosslink {
             epoch: spec.genesis_epoch,
             crosslink_data_root: spec.zero_hash,
         };
 
-        Ok(BeaconState {
+        BeaconState {
             /*
              * Misc
              */
@@ -188,52 +186,12 @@ impl BeaconState {
              */
             cache_index_offset: 0,
             caches: vec![EpochCache::empty(); CACHED_EPOCHS],
-        })
-    }
-
-    /// Produce the first state of the Beacon Chain.
-    pub fn genesis(
-        genesis_time: u64,
-        initial_validator_deposits: Vec<Deposit>,
-        latest_eth1_data: Eth1Data,
-        spec: &ChainSpec,
-    ) -> Result<BeaconState, Error> {
-        let mut genesis_state =
-            BeaconState::genesis_without_validators(genesis_time, latest_eth1_data, spec)?;
-
-        debug!("Processing genesis deposits...");
-
-        let deposit_data = initial_validator_deposits
-            .par_iter()
-            .map(|deposit| &deposit.deposit_data)
-            .collect();
-
-        genesis_state.process_deposits(deposit_data, spec);
-
-        trace!("Processed genesis deposits.");
-
-        for validator_index in 0..genesis_state.validator_registry.len() {
-            if genesis_state.get_effective_balance(validator_index, spec) >= spec.max_deposit_amount
-            {
-                genesis_state.activate_validator(validator_index, true, spec);
-            }
         }
-
-        genesis_state.deposit_index = initial_validator_deposits.len() as u64;
-
-        let genesis_active_index_root = hash_tree_root(get_active_validator_indices(
-            &genesis_state.validator_registry,
-            spec.genesis_epoch,
-        ));
-        genesis_state.latest_active_index_roots =
-            vec![genesis_active_index_root; spec.latest_active_index_roots_length];
-        genesis_state.current_shuffling_seed =
-            genesis_state.generate_seed(spec.genesis_epoch, spec)?;
-
-        Ok(genesis_state)
     }
 
     /// Returns the `hash_tree_root` of the state.
+    ///
+    /// Spec v0.4.0
     pub fn canonical_root(&self) -> Hash256 {
         Hash256::from_slice(&self.hash_tree_root()[..])
     }
@@ -541,12 +499,14 @@ impl BeaconState {
             return Err(Error::InvalidBitfield);
         }
 
-        let mut participants = vec![];
+        let mut participants = Vec::with_capacity(committee.len());
         for (i, validator_index) in committee.iter().enumerate() {
-            if bitfield.get(i).unwrap() {
-                participants.push(*validator_index);
+            match bitfield.get(i) {
+                Ok(bit) if bit == true => participants.push(*validator_index),
+                _ => {}
             }
         }
+        participants.shrink_to_fit();
 
         Ok(participants)
     }
@@ -598,10 +558,8 @@ impl BeaconState {
 
         for deposit_data in deposits {
             let result = self.process_deposit(
-                deposit_data.deposit_input.pubkey.clone(),
+                deposit_data.deposit_input.clone(),
                 deposit_data.amount,
-                deposit_data.deposit_input.proof_of_possession.clone(),
-                deposit_data.deposit_input.withdrawal_credentials,
                 Some(&pubkey_map),
                 spec,
             );
@@ -618,24 +576,30 @@ impl BeaconState {
     /// this hashmap, each call to `process_deposits` requires an iteration though
     /// `self.validator_registry`. This becomes highly inefficient at scale.
     ///
+    /// TODO: this function also exists in a more optimal form in the `state_processing` crate as
+    /// `process_deposits`; unify these two functions.
+    ///
     /// Spec v0.4.0
     pub fn process_deposit(
         &mut self,
-        pubkey: PublicKey,
+        deposit_input: DepositInput,
         amount: u64,
-        proof_of_possession: Signature,
-        withdrawal_credentials: Hash256,
         pubkey_map: Option<&HashMap<PublicKey, usize>>,
         spec: &ChainSpec,
     ) -> Result<usize, ()> {
-        // TODO: update proof of possession to function written above (
-        // requires bls::create_proof_of_possession to be updated
-        //
-        // https://github.com/sigp/lighthouse/issues/239
-        if !verify_proof_of_possession(&proof_of_possession, &pubkey) {
+        let proof_is_valid = deposit_input.proof_of_possession.verify(
+            &deposit_input.signed_root(),
+            spec.get_domain(self.current_epoch(&spec), Domain::Deposit, &self.fork),
+            &deposit_input.pubkey,
+        );
+
+        if !proof_is_valid {
             return Err(());
         }
 
+        let pubkey = deposit_input.pubkey.clone();
+        let withdrawal_credentials = deposit_input.withdrawal_credentials.clone();
+
         let validator_index = if let Some(pubkey_map) = pubkey_map {
             pubkey_map.get(&pubkey).and_then(|i| Some(*i))
         } else {
@@ -1063,33 +1027,6 @@ impl BeaconState {
         self.validator_registry_update_epoch = current_epoch;
     }
 
-    /// Confirm validator owns PublicKey
-    ///
-    /// Spec v0.4.0
-    pub fn validate_proof_of_possession(
-        &self,
-        pubkey: PublicKey,
-        proof_of_possession: Signature,
-        withdrawal_credentials: Hash256,
-        spec: &ChainSpec,
-    ) -> bool {
-        let proof_of_possession_data = DepositInput {
-            pubkey: pubkey.clone(),
-            withdrawal_credentials,
-            proof_of_possession: Signature::empty_signature(),
-        };
-
-        proof_of_possession.verify(
-            &proof_of_possession_data.hash_tree_root(),
-            spec.get_domain(
-                self.slot.epoch(spec.slots_per_epoch),
-                Domain::Deposit,
-                &self.fork,
-            ),
-            &pubkey,
-        )
-    }
-
     /// Iterate through the validator registry and eject active validators with balance below
     /// ``EJECTION_BALANCE``.
     ///
@@ -1161,10 +1098,6 @@ impl BeaconState {
     }
 }
 
-fn hash_tree_root<T: TreeHash>(input: Vec<T>) -> Hash256 {
-    Hash256::from_slice(&input.hash_tree_root()[..])
-}
-
 impl Encodable for BeaconState {
     fn ssz_append(&self, s: &mut SszStream) {
         s.append(&self.slot);
diff --git a/eth2/types/src/beacon_state/builder.rs b/eth2/types/src/beacon_state/builder.rs
index 0be297db7..22ca3e622 100644
--- a/eth2/types/src/beacon_state/builder.rs
+++ b/eth2/types/src/beacon_state/builder.rs
@@ -1,263 +1,99 @@
+use super::BeaconStateError;
+use crate::validator_registry::get_active_validator_indices;
 use crate::*;
-use bls::create_proof_of_possession;
+use rayon::prelude::*;
+use ssz::TreeHash;
 
-/// Builds a `BeaconState` for use in testing or benchmarking.
+/// Builds a `BeaconState` for use in production.
 ///
-/// Building the `BeaconState` is a three step processes:
+/// This struct should _not_ be modified for use in testing scenarios. Use `TestingBeaconStateBuilder` for that purpose.
 ///
-/// 1. Create a new `BeaconStateBuilder`.
-/// 2. Call `Self::build()` or `Self::build_fast()` generate a  `BeaconState`.
-/// 3. (Optional) Use builder functions to modify the `BeaconState`.
-/// 4. Call `Self::cloned_state()` to obtain a `BeaconState` cloned from this struct.
-///
-/// Step (2) happens prior to step (3) because some functionality requires an existing
-/// `BeaconState`.
-///
-/// Step (4) produces a clone of the BeaconState and doesn't consume the `BeaconStateBuilder` to
-/// allow access to `self.keypairs` and `self.spec`.
+/// This struct should remain safe and sensible for production usage.
 pub struct BeaconStateBuilder {
-    pub validator_count: usize,
-    pub state: Option<BeaconState>,
-    pub genesis_time: u64,
-    pub latest_eth1_data: Eth1Data,
-    pub spec: ChainSpec,
-    pub keypairs: Vec<Keypair>,
+    pub state: BeaconState,
 }
 
 impl BeaconStateBuilder {
     /// Create a new builder with the given number of validators.
-    pub fn new(validator_count: usize) -> Self {
-        let genesis_time = 10_000_000;
-
-        let latest_eth1_data = Eth1Data {
-            deposit_root: Hash256::zero(),
-            block_hash: Hash256::zero(),
-        };
-
-        let spec = ChainSpec::foundation();
-
+    ///
+    /// Spec v0.4.0
+    pub fn new(genesis_time: u64, latest_eth1_data: Eth1Data, spec: &ChainSpec) -> Self {
         Self {
-            validator_count,
-            state: None,
-            genesis_time,
-            latest_eth1_data,
-            spec,
-            keypairs: vec![],
+            state: BeaconState::genesis(genesis_time, latest_eth1_data, spec),
         }
     }
 
-    /// Builds a `BeaconState` using the `BeaconState::genesis(..)` function.
+    /// Process deposit objects.
     ///
-    /// Each validator is assigned a unique, randomly-generated keypair and all
-    /// proof-of-possessions are verified during genesis.
-    pub fn build(&mut self) -> Result<(), BeaconStateError> {
-        self.keypairs = (0..self.validator_count)
-            .collect::<Vec<usize>>()
-            .iter()
-            .map(|_| Keypair::random())
+    /// Spec v0.4.0
+    pub fn process_initial_deposits(
+        &mut self,
+        initial_validator_deposits: &[Deposit],
+        spec: &ChainSpec,
+    ) {
+        let deposit_data = initial_validator_deposits
+            .par_iter()
+            .map(|deposit| &deposit.deposit_data)
             .collect();
 
-        let initial_validator_deposits = self
-            .keypairs
-            .iter()
-            .map(|keypair| Deposit {
-                branch: vec![], // branch verification is not specified.
-                index: 0,       // index verification is not specified.
-                deposit_data: DepositData {
-                    amount: 32_000_000_000, // 32 ETH (in Gwei)
-                    timestamp: self.genesis_time - 1,
-                    deposit_input: DepositInput {
-                        pubkey: keypair.pk.clone(),
-                        withdrawal_credentials: Hash256::zero(), // Withdrawal not possible.
-                        proof_of_possession: create_proof_of_possession(&keypair),
-                    },
-                },
-            })
-            .collect();
+        self.state.process_deposits(deposit_data, spec);
 
-        let state = BeaconState::genesis(
-            self.genesis_time,
-            initial_validator_deposits,
-            self.latest_eth1_data.clone(),
-            &self.spec,
-        )?;
+        self.activate_genesis_validators(spec);
 
-        self.state = Some(state);
-
-        Ok(())
+        self.state.deposit_index = initial_validator_deposits.len() as u64;
     }
 
-    /// Builds a `BeaconState` using the `BeaconState::genesis(..)` function, without supplying any
-    /// validators. Instead validators are added to the state post-genesis.
-    ///
-    /// One keypair is randomly generated and all validators are assigned this same keypair.
-    /// Proof-of-possessions are not created (or validated).
-    ///
-    /// This function runs orders of magnitude faster than `Self::build()`, however it will be
-    /// erroneous for functions which use a validators public key as an identifier (e.g.,
-    /// deposits).
-    pub fn build_fast(&mut self) -> Result<(), BeaconStateError> {
-        let common_keypair = Keypair::random();
-
-        let mut validator_registry = Vec::with_capacity(self.validator_count);
-        let mut validator_balances = Vec::with_capacity(self.validator_count);
-        self.keypairs = Vec::with_capacity(self.validator_count);
-
-        for _ in 0..self.validator_count {
-            self.keypairs.push(common_keypair.clone());
-            validator_balances.push(32_000_000_000);
-            validator_registry.push(Validator {
-                pubkey: common_keypair.pk.clone(),
-                withdrawal_credentials: Hash256::zero(),
-                activation_epoch: self.spec.genesis_epoch,
-                ..Validator::default()
-            })
-        }
-
-        let state = BeaconState {
-            validator_registry,
-            validator_balances,
-            ..BeaconState::genesis(
-                self.genesis_time,
-                vec![],
-                self.latest_eth1_data.clone(),
-                &self.spec,
-            )?
-        };
-
-        self.state = Some(state);
-
-        Ok(())
-    }
-
-    /// Sets the `BeaconState` to be in the last slot of the given epoch.
-    ///
-    /// Sets all justification/finalization parameters to be be as "perfect" as possible (i.e.,
-    /// highest justified and finalized slots, full justification bitfield, etc).
-    pub fn teleport_to_end_of_epoch(&mut self, epoch: Epoch) {
-        let state = self.state.as_mut().expect("Genesis required");
-
-        let slot = epoch.end_slot(self.spec.slots_per_epoch);
-
-        state.slot = slot;
-        state.validator_registry_update_epoch = epoch - 1;
-
-        state.previous_shuffling_epoch = epoch - 1;
-        state.current_shuffling_epoch = epoch;
-
-        state.previous_shuffling_seed = Hash256::from_low_u64_le(0);
-        state.current_shuffling_seed = Hash256::from_low_u64_le(1);
-
-        state.previous_justified_epoch = epoch - 2;
-        state.justified_epoch = epoch - 1;
-        state.justification_bitfield = u64::max_value();
-        state.finalized_epoch = epoch - 1;
-    }
-
-    /// Creates a full set of attestations for the `BeaconState`. Each attestation has full
-    /// participation from its committee and references the expected beacon_block hashes.
-    ///
-    /// These attestations should be fully conducive to justification and finalization.
-    pub fn insert_attestations(&mut self) {
-        let state = self.state.as_mut().expect("Genesis required");
-
-        state
-            .build_epoch_cache(RelativeEpoch::Previous, &self.spec)
-            .unwrap();
-        state
-            .build_epoch_cache(RelativeEpoch::Current, &self.spec)
-            .unwrap();
-
-        let current_epoch = state.current_epoch(&self.spec);
-        let previous_epoch = state.previous_epoch(&self.spec);
-        let current_epoch_depth =
-            (state.slot - current_epoch.end_slot(self.spec.slots_per_epoch)).as_usize();
-
-        let previous_epoch_slots = previous_epoch.slot_iter(self.spec.slots_per_epoch);
-        let current_epoch_slots = current_epoch
-            .slot_iter(self.spec.slots_per_epoch)
-            .take(current_epoch_depth);
-
-        for slot in previous_epoch_slots.chain(current_epoch_slots) {
-            let committees = state
-                .get_crosslink_committees_at_slot(slot, &self.spec)
-                .unwrap()
-                .clone();
-
-            for (committee, shard) in committees {
-                state
-                    .latest_attestations
-                    .push(committee_to_pending_attestation(
-                        state, &committee, shard, slot, &self.spec,
-                    ))
+    fn activate_genesis_validators(&mut self, spec: &ChainSpec) {
+        for validator_index in 0..self.state.validator_registry.len() {
+            if self.state.get_effective_balance(validator_index, spec) >= spec.max_deposit_amount {
+                self.state.activate_validator(validator_index, true, spec);
             }
         }
     }
 
-    /// Returns a cloned `BeaconState`.
-    pub fn cloned_state(&self) -> BeaconState {
-        self.state.as_ref().expect("Genesis required").clone()
-    }
-}
-
-/// Builds a valid PendingAttestation with full participation for some committee.
-fn committee_to_pending_attestation(
-    state: &BeaconState,
-    committee: &[usize],
-    shard: u64,
-    slot: Slot,
-    spec: &ChainSpec,
-) -> PendingAttestation {
-    let current_epoch = state.current_epoch(spec);
-    let previous_epoch = state.previous_epoch(spec);
-
-    let mut aggregation_bitfield = Bitfield::new();
-    let mut custody_bitfield = Bitfield::new();
-
-    for (i, _) in committee.iter().enumerate() {
-        aggregation_bitfield.set(i, true);
-        custody_bitfield.set(i, true);
-    }
-
-    let is_previous_epoch =
-        state.slot.epoch(spec.slots_per_epoch) != slot.epoch(spec.slots_per_epoch);
-
-    let justified_epoch = if is_previous_epoch {
-        state.previous_justified_epoch
-    } else {
-        state.justified_epoch
-    };
-
-    let epoch_boundary_root = if is_previous_epoch {
-        *state
-            .get_block_root(previous_epoch.start_slot(spec.slots_per_epoch), spec)
-            .unwrap()
-    } else {
-        *state
-            .get_block_root(current_epoch.start_slot(spec.slots_per_epoch), spec)
-            .unwrap()
-    };
-
-    let justified_block_root = *state
-        .get_block_root(justified_epoch.start_slot(spec.slots_per_epoch), &spec)
-        .unwrap();
-
-    PendingAttestation {
-        aggregation_bitfield,
-        data: AttestationData {
-            slot,
-            shard,
-            beacon_block_root: *state.get_block_root(slot, spec).unwrap(),
-            epoch_boundary_root,
-            crosslink_data_root: Hash256::zero(),
-            latest_crosslink: Crosslink {
-                epoch: slot.epoch(spec.slots_per_epoch),
-                crosslink_data_root: Hash256::zero(),
-            },
-            justified_epoch,
-            justified_block_root,
-        },
-        custody_bitfield,
-        inclusion_slot: slot,
+    /// Instantiate the validator registry from a YAML file.
+    ///
+    /// This skips a lot of signing and verification, useful if signing and verification has been
+    /// completed previously.
+    ///
+    /// Spec v0.4.0
+    pub fn import_existing_validators(
+        &mut self,
+        validators: Vec<Validator>,
+        initial_balances: Vec<u64>,
+        deposit_index: u64,
+        spec: &ChainSpec,
+    ) {
+        self.state.validator_registry = validators;
+
+        assert_eq!(
+            self.state.validator_registry.len(),
+            initial_balances.len(),
+            "Not enough balances for validators"
+        );
+
+        self.state.validator_balances = initial_balances;
+
+        self.activate_genesis_validators(spec);
+
+        self.state.deposit_index = deposit_index;
+    }
+
+    /// Updates the final state variables and returns a fully built genesis state.
+    ///
+    /// Spec v0.4.0
+    pub fn build(mut self, spec: &ChainSpec) -> Result<BeaconState, BeaconStateError> {
+        let genesis_active_index_root =
+            get_active_validator_indices(&self.state.validator_registry, spec.genesis_epoch)
+                .hash_tree_root();
+
+        self.state.latest_active_index_roots = vec![
+            Hash256::from_slice(&genesis_active_index_root);
+            spec.latest_active_index_roots_length
+        ];
+
+        self.state.current_shuffling_seed = self.state.generate_seed(spec.genesis_epoch, spec)?;
+
+        Ok(self.state)
     }
 }
diff --git a/eth2/types/src/beacon_state/helpers.rs b/eth2/types/src/beacon_state/helpers.rs
index c93b16f76..adae7bab4 100644
--- a/eth2/types/src/beacon_state/helpers.rs
+++ b/eth2/types/src/beacon_state/helpers.rs
@@ -11,7 +11,7 @@ pub fn verify_bitfield_length(bitfield: &Bitfield, committee_size: usize) -> boo
     }
 
     for i in committee_size..(bitfield.num_bytes() * 8) {
-        if bitfield.get(i).expect("Impossible due to previous check.") {
+        if bitfield.get(i).unwrap_or(false) {
             return false;
         }
     }
diff --git a/eth2/types/src/beacon_state/tests.rs b/eth2/types/src/beacon_state/tests.rs
index 40bfd146c..61f3c03b0 100644
--- a/eth2/types/src/beacon_state/tests.rs
+++ b/eth2/types/src/beacon_state/tests.rs
@@ -1,29 +1,20 @@
 #![cfg(test)]
 
 use super::*;
+use crate::test_utils::TestingBeaconStateBuilder;
 use crate::test_utils::{SeedableRng, TestRandom, XorShiftRng};
 use crate::{BeaconState, ChainSpec};
 use ssz::{ssz_encode, Decodable};
 
-#[test]
-pub fn can_produce_genesis_block() {
-    let mut builder = BeaconStateBuilder::new(2);
-    builder.build().unwrap();
-}
-
 /// Tests that `get_attestation_participants` is consistent with the result of
 /// get_crosslink_committees_at_slot` with a full bitfield.
 #[test]
 pub fn get_attestation_participants_consistency() {
     let mut rng = XorShiftRng::from_seed([42; 16]);
 
-    let mut builder = BeaconStateBuilder::new(8);
-    builder.spec = ChainSpec::few_validators();
-
-    builder.build().unwrap();
-
-    let mut state = builder.cloned_state();
-    let spec = builder.spec.clone();
+    let spec = ChainSpec::few_validators();
+    let builder = TestingBeaconStateBuilder::from_deterministic_keypairs(8, &spec);
+    let (mut state, _keypairs) = builder.build();
 
     state
         .build_epoch_cache(RelativeEpoch::Previous, &spec)
diff --git a/eth2/types/src/deposit.rs b/eth2/types/src/deposit.rs
index 2e69ea599..91c6ef2ac 100644
--- a/eth2/types/src/deposit.rs
+++ b/eth2/types/src/deposit.rs
@@ -1,14 +1,14 @@
 use super::{DepositData, Hash256};
 use crate::test_utils::TestRandom;
 use rand::RngCore;
-use serde_derive::Serialize;
+use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode, TreeHash};
 use test_random_derive::TestRandom;
 
 /// A deposit to potentially become a beacon chain validator.
 ///
 /// Spec v0.4.0
-#[derive(Debug, PartialEq, Clone, Serialize, Encode, Decode, TreeHash, TestRandom)]
+#[derive(Debug, PartialEq, Clone, Serialize, Deserialize, Encode, Decode, TreeHash, TestRandom)]
 pub struct Deposit {
     pub branch: Vec<Hash256>,
     pub index: u64,
diff --git a/eth2/types/src/deposit_data.rs b/eth2/types/src/deposit_data.rs
index 1eb2722a9..61b82f4b3 100644
--- a/eth2/types/src/deposit_data.rs
+++ b/eth2/types/src/deposit_data.rs
@@ -1,14 +1,14 @@
 use super::DepositInput;
 use crate::test_utils::TestRandom;
 use rand::RngCore;
-use serde_derive::Serialize;
+use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode, TreeHash};
 use test_random_derive::TestRandom;
 
 /// Data generated by the deposit contract.
 ///
 /// Spec v0.4.0
-#[derive(Debug, PartialEq, Clone, Serialize, Encode, Decode, TreeHash, TestRandom)]
+#[derive(Debug, PartialEq, Clone, Serialize, Deserialize, Encode, Decode, TreeHash, TestRandom)]
 pub struct DepositData {
     pub amount: u64,
     pub timestamp: u64,
diff --git a/eth2/types/src/deposit_input.rs b/eth2/types/src/deposit_input.rs
index c4c79c3d1..1b506894d 100644
--- a/eth2/types/src/deposit_input.rs
+++ b/eth2/types/src/deposit_input.rs
@@ -1,21 +1,68 @@
-use super::Hash256;
 use crate::test_utils::TestRandom;
-use bls::{PublicKey, Signature};
+use crate::*;
+use bls::{Keypair, PublicKey, Signature};
 use rand::RngCore;
-use serde_derive::Serialize;
-use ssz_derive::{Decode, Encode, TreeHash};
+use serde_derive::{Deserialize, Serialize};
+use ssz::{SignedRoot, TreeHash};
+use ssz_derive::{Decode, Encode, SignedRoot, TreeHash};
 use test_random_derive::TestRandom;
 
 /// The data supplied by the user to the deposit contract.
 ///
 /// Spec v0.4.0
-#[derive(Debug, PartialEq, Clone, Serialize, Encode, Decode, TreeHash, TestRandom)]
+#[derive(
+    Debug,
+    PartialEq,
+    Clone,
+    Serialize,
+    Deserialize,
+    Encode,
+    Decode,
+    SignedRoot,
+    TreeHash,
+    TestRandom,
+)]
 pub struct DepositInput {
     pub pubkey: PublicKey,
     pub withdrawal_credentials: Hash256,
     pub proof_of_possession: Signature,
 }
 
+impl DepositInput {
+    /// Generate the 'proof_of_posession' signature for a given DepositInput details.
+    ///
+    /// Spec v0.4.0
+    pub fn create_proof_of_possession(
+        keypair: &Keypair,
+        withdrawal_credentials: &Hash256,
+        domain: u64,
+    ) -> Signature {
+        let signable_deposit_input = DepositInput {
+            pubkey: keypair.pk.clone(),
+            withdrawal_credentials: withdrawal_credentials.clone(),
+            proof_of_possession: Signature::empty_signature(),
+        };
+        let msg = signable_deposit_input.signed_root();
+
+        Signature::new(msg.as_slice(), domain, &keypair.sk)
+    }
+
+    /// Verify that proof-of-possession is valid.
+    ///
+    /// Spec v0.4.0
+    pub fn validate_proof_of_possession(
+        &self,
+        epoch: Epoch,
+        fork: &Fork,
+        spec: &ChainSpec,
+    ) -> bool {
+        let msg = self.signed_root();
+        let domain = spec.get_domain(epoch, Domain::Deposit, fork);
+
+        self.proof_of_possession.verify(&msg, domain, &self.pubkey)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
diff --git a/eth2/types/src/lib.rs b/eth2/types/src/lib.rs
index 9bf60f2c9..76fcb43ed 100644
--- a/eth2/types/src/lib.rs
+++ b/eth2/types/src/lib.rs
@@ -72,4 +72,4 @@ pub type AttesterMap = HashMap<(u64, u64), Vec<usize>>;
 /// Maps a slot to a block proposer.
 pub type ProposerMap = HashMap<u64, usize>;
 
-pub use bls::{AggregatePublicKey, AggregateSignature, Keypair, PublicKey, Signature};
+pub use bls::{AggregatePublicKey, AggregateSignature, Keypair, PublicKey, SecretKey, Signature};
diff --git a/eth2/types/src/proposer_slashing.rs b/eth2/types/src/proposer_slashing.rs
index f86e7f3a8..394c55a01 100644
--- a/eth2/types/src/proposer_slashing.rs
+++ b/eth2/types/src/proposer_slashing.rs
@@ -5,10 +5,6 @@ use serde_derive::Serialize;
 use ssz_derive::{Decode, Encode, TreeHash};
 use test_random_derive::TestRandom;
 
-mod builder;
-
-pub use builder::ProposerSlashingBuilder;
-
 /// Two conflicting proposals from the same proposer (validator).
 ///
 /// Spec v0.4.0
diff --git a/eth2/types/src/slot_epoch.rs b/eth2/types/src/slot_epoch.rs
index 7753027a6..2af7f5196 100644
--- a/eth2/types/src/slot_epoch.rs
+++ b/eth2/types/src/slot_epoch.rs
@@ -12,7 +12,7 @@ use crate::slot_height::SlotHeight;
 /// may lead to programming errors which are not detected by the compiler.
 use crate::test_utils::TestRandom;
 use rand::RngCore;
-use serde_derive::Serialize;
+use serde_derive::{Deserialize, Serialize};
 use slog;
 use ssz::{hash, ssz_encode, Decodable, DecodeError, Encodable, SszStream, TreeHash};
 use std::cmp::{Ord, Ordering};
@@ -21,10 +21,10 @@ use std::hash::{Hash, Hasher};
 use std::iter::Iterator;
 use std::ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Rem, Sub, SubAssign};
 
-#[derive(Eq, Debug, Clone, Copy, Default, Serialize)]
+#[derive(Eq, Debug, Clone, Copy, Default, Serialize, Deserialize)]
 pub struct Slot(u64);
 
-#[derive(Eq, Debug, Clone, Copy, Default, Serialize)]
+#[derive(Eq, Debug, Clone, Copy, Default, Serialize, Deserialize)]
 pub struct Epoch(u64);
 
 impl_common!(Slot);
diff --git a/eth2/types/src/test_utils/generate_deterministic_keypairs.rs b/eth2/types/src/test_utils/generate_deterministic_keypairs.rs
new file mode 100644
index 000000000..f2ce8709e
--- /dev/null
+++ b/eth2/types/src/test_utils/generate_deterministic_keypairs.rs
@@ -0,0 +1,30 @@
+use crate::*;
+use int_to_bytes::int_to_bytes48;
+use log::debug;
+use rayon::prelude::*;
+
+/// Generates `validator_count` keypairs where the secret key is the index of the
+/// validator.
+///
+/// For example, the first validator has a secret key of `int_to_bytes48(1)`, the second has
+/// `int_to_bytes48(2)` and so on. (We skip `0` as it generates a weird looking public key and is
+/// probably invalid).
+pub fn generate_deterministic_keypairs(validator_count: usize) -> Vec<Keypair> {
+    debug!(
+        "Generating {} deterministic validator keypairs...",
+        validator_count
+    );
+
+    let keypairs: Vec<Keypair> = (0..validator_count)
+        .collect::<Vec<usize>>()
+        .par_iter()
+        .map(|&i| {
+            let secret = int_to_bytes48(i as u64 + 1);
+            let sk = SecretKey::from_bytes(&secret).unwrap();
+            let pk = PublicKey::from_secret_key(&sk);
+            Keypair { sk, pk }
+        })
+        .collect();
+
+    keypairs
+}
diff --git a/eth2/types/src/test_utils/keypairs_file.rs b/eth2/types/src/test_utils/keypairs_file.rs
new file mode 100644
index 000000000..a1ea4d928
--- /dev/null
+++ b/eth2/types/src/test_utils/keypairs_file.rs
@@ -0,0 +1,128 @@
+use crate::*;
+use rayon::prelude::*;
+use std::fs::File;
+use std::io::{Error, ErrorKind, Read, Write};
+use std::path::Path;
+
+pub const PUBLIC_KEY_BYTES_LEN: usize = 96;
+pub const SECRET_KEY_BYTES_LEN: usize = 48;
+
+pub const BATCH_SIZE: usize = 1_000; // ~15MB
+
+pub const KEYPAIR_BYTES_LEN: usize = PUBLIC_KEY_BYTES_LEN + SECRET_KEY_BYTES_LEN;
+pub const BATCH_BYTE_LEN: usize = KEYPAIR_BYTES_LEN * BATCH_SIZE;
+
+/// Defines a trait that allows reading/writing a vec of `Keypair` from/to a file.
+pub trait KeypairsFile {
+    /// Write to file, without guaranteeing interoperability with other clients.
+    fn to_raw_file(&self, path: &Path, keypairs: &[Keypair]) -> Result<(), Error>;
+    /// Read from file, without guaranteeing interoperability with other clients.
+    fn from_raw_file(path: &Path, count: usize) -> Result<Vec<Keypair>, Error>;
+}
+
+impl KeypairsFile for Vec<Keypair> {
+    /// Write the keypairs to file, using the fastest possible method without guaranteeing
+    /// interoperability with other clients.
+    fn to_raw_file(&self, path: &Path, keypairs: &[Keypair]) -> Result<(), Error> {
+        let mut keypairs_file = File::create(path)?;
+
+        for keypair_batch in keypairs.chunks(BATCH_SIZE) {
+            let mut buf = Vec::with_capacity(BATCH_BYTE_LEN);
+
+            for keypair in keypair_batch {
+                buf.append(&mut keypair.sk.as_raw().as_bytes());
+                buf.append(&mut keypair.pk.clone().as_uncompressed_bytes());
+            }
+
+            keypairs_file.write_all(&buf)?;
+        }
+
+        Ok(())
+    }
+
+    /// Read the keypairs from file, using the fastest possible method without guaranteeing
+    /// interoperability with other clients.
+    fn from_raw_file(path: &Path, count: usize) -> Result<Vec<Keypair>, Error> {
+        let mut keypairs_file = File::open(path)?;
+
+        let mut keypairs = Vec::with_capacity(count);
+
+        let indices: Vec<usize> = (0..count).collect();
+
+        for batch in indices.chunks(BATCH_SIZE) {
+            let mut buf = vec![0; batch.len() * KEYPAIR_BYTES_LEN];
+            keypairs_file.read_exact(&mut buf)?;
+
+            let mut keypair_batch = batch
+                .par_iter()
+                .enumerate()
+                .map(|(i, _)| {
+                    let sk_start = i * KEYPAIR_BYTES_LEN;
+                    let sk_end = sk_start + SECRET_KEY_BYTES_LEN;
+                    let sk = SecretKey::from_bytes(&buf[sk_start..sk_end])
+                        .map_err(|_| Error::new(ErrorKind::Other, "Invalid SecretKey bytes"))
+                        .unwrap();
+
+                    let pk_start = sk_end;
+                    let pk_end = pk_start + PUBLIC_KEY_BYTES_LEN;
+                    let pk = PublicKey::from_uncompressed_bytes(&buf[pk_start..pk_end])
+                        .map_err(|_| Error::new(ErrorKind::Other, "Invalid PublicKey bytes"))
+                        .unwrap();
+
+                    Keypair { sk, pk }
+                })
+                .collect();
+
+            keypairs.append(&mut keypair_batch);
+        }
+
+        Ok(keypairs)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use rand::{distributions::Alphanumeric, thread_rng, Rng};
+    use std::fs::remove_file;
+
+    fn random_keypairs(n: usize) -> Vec<Keypair> {
+        (0..n).into_par_iter().map(|_| Keypair::random()).collect()
+    }
+
+    fn random_tmp_file() -> String {
+        let mut rng = thread_rng();
+
+        rng.sample_iter(&Alphanumeric).take(7).collect()
+    }
+
+    #[test]
+    #[ignore]
+    fn read_write_consistency_small_batch() {
+        let num_keypairs = 10;
+        let keypairs = random_keypairs(num_keypairs);
+
+        let keypairs_path = Path::new("/tmp").join(random_tmp_file());
+        keypairs.to_raw_file(&keypairs_path, &keypairs).unwrap();
+
+        let decoded = Vec::from_raw_file(&keypairs_path, num_keypairs).unwrap();
+        remove_file(keypairs_path).unwrap();
+
+        assert_eq!(keypairs, decoded);
+    }
+
+    #[test]
+    #[ignore]
+    fn read_write_consistency_big_batch() {
+        let num_keypairs = BATCH_SIZE + 1;
+        let keypairs = random_keypairs(num_keypairs);
+
+        let keypairs_path = Path::new("/tmp").join(random_tmp_file());
+        keypairs.to_raw_file(&keypairs_path, &keypairs).unwrap();
+
+        let decoded = Vec::from_raw_file(&keypairs_path, num_keypairs).unwrap();
+        remove_file(keypairs_path).unwrap();
+
+        assert_eq!(keypairs, decoded);
+    }
+}
diff --git a/eth2/types/src/test_utils/mod.rs b/eth2/types/src/test_utils/mod.rs
index 82e060fca..6fdbe53ad 100644
--- a/eth2/types/src/test_utils/mod.rs
+++ b/eth2/types/src/test_utils/mod.rs
@@ -1,55 +1,24 @@
-use rand::RngCore;
+mod generate_deterministic_keypairs;
+mod keypairs_file;
+mod test_random;
+mod testing_attestation_builder;
+mod testing_attester_slashing_builder;
+mod testing_beacon_block_builder;
+mod testing_beacon_state_builder;
+mod testing_deposit_builder;
+mod testing_proposer_slashing_builder;
+mod testing_transfer_builder;
+mod testing_voluntary_exit_builder;
 
+pub use generate_deterministic_keypairs::generate_deterministic_keypairs;
+pub use keypairs_file::KeypairsFile;
 pub use rand::{prng::XorShiftRng, SeedableRng};
-
-pub mod address;
-pub mod aggregate_signature;
-pub mod bitfield;
-pub mod hash256;
-pub mod public_key;
-pub mod secret_key;
-pub mod signature;
-
-pub trait TestRandom<T>
-where
-    T: RngCore,
-{
-    fn random_for_test(rng: &mut T) -> Self;
-}
-
-impl<T: RngCore> TestRandom<T> for bool {
-    fn random_for_test(rng: &mut T) -> Self {
-        (rng.next_u32() % 2) == 1
-    }
-}
-
-impl<T: RngCore> TestRandom<T> for u64 {
-    fn random_for_test(rng: &mut T) -> Self {
-        rng.next_u64()
-    }
-}
-
-impl<T: RngCore> TestRandom<T> for u32 {
-    fn random_for_test(rng: &mut T) -> Self {
-        rng.next_u32()
-    }
-}
-
-impl<T: RngCore> TestRandom<T> for usize {
-    fn random_for_test(rng: &mut T) -> Self {
-        rng.next_u32() as usize
-    }
-}
-
-impl<T: RngCore, U> TestRandom<T> for Vec<U>
-where
-    U: TestRandom<T>,
-{
-    fn random_for_test(rng: &mut T) -> Self {
-        vec![
-            <U>::random_for_test(rng),
-            <U>::random_for_test(rng),
-            <U>::random_for_test(rng),
-        ]
-    }
-}
+pub use test_random::TestRandom;
+pub use testing_attestation_builder::TestingAttestationBuilder;
+pub use testing_attester_slashing_builder::TestingAttesterSlashingBuilder;
+pub use testing_beacon_block_builder::TestingBeaconBlockBuilder;
+pub use testing_beacon_state_builder::{keypairs_path, TestingBeaconStateBuilder};
+pub use testing_deposit_builder::TestingDepositBuilder;
+pub use testing_proposer_slashing_builder::TestingProposerSlashingBuilder;
+pub use testing_transfer_builder::TestingTransferBuilder;
+pub use testing_voluntary_exit_builder::TestingVoluntaryExitBuilder;
diff --git a/eth2/types/src/test_utils/test_random.rs b/eth2/types/src/test_utils/test_random.rs
new file mode 100644
index 000000000..3b172463e
--- /dev/null
+++ b/eth2/types/src/test_utils/test_random.rs
@@ -0,0 +1,53 @@
+use rand::RngCore;
+
+mod address;
+mod aggregate_signature;
+mod bitfield;
+mod hash256;
+mod public_key;
+mod secret_key;
+mod signature;
+
+pub trait TestRandom<T>
+where
+    T: RngCore,
+{
+    fn random_for_test(rng: &mut T) -> Self;
+}
+
+impl<T: RngCore> TestRandom<T> for bool {
+    fn random_for_test(rng: &mut T) -> Self {
+        (rng.next_u32() % 2) == 1
+    }
+}
+
+impl<T: RngCore> TestRandom<T> for u64 {
+    fn random_for_test(rng: &mut T) -> Self {
+        rng.next_u64()
+    }
+}
+
+impl<T: RngCore> TestRandom<T> for u32 {
+    fn random_for_test(rng: &mut T) -> Self {
+        rng.next_u32()
+    }
+}
+
+impl<T: RngCore> TestRandom<T> for usize {
+    fn random_for_test(rng: &mut T) -> Self {
+        rng.next_u32() as usize
+    }
+}
+
+impl<T: RngCore, U> TestRandom<T> for Vec<U>
+where
+    U: TestRandom<T>,
+{
+    fn random_for_test(rng: &mut T) -> Self {
+        vec![
+            <U>::random_for_test(rng),
+            <U>::random_for_test(rng),
+            <U>::random_for_test(rng),
+        ]
+    }
+}
diff --git a/eth2/types/src/test_utils/address.rs b/eth2/types/src/test_utils/test_random/address.rs
similarity index 100%
rename from eth2/types/src/test_utils/address.rs
rename to eth2/types/src/test_utils/test_random/address.rs
diff --git a/eth2/types/src/test_utils/aggregate_signature.rs b/eth2/types/src/test_utils/test_random/aggregate_signature.rs
similarity index 100%
rename from eth2/types/src/test_utils/aggregate_signature.rs
rename to eth2/types/src/test_utils/test_random/aggregate_signature.rs
diff --git a/eth2/types/src/test_utils/bitfield.rs b/eth2/types/src/test_utils/test_random/bitfield.rs
similarity index 90%
rename from eth2/types/src/test_utils/bitfield.rs
rename to eth2/types/src/test_utils/test_random/bitfield.rs
index 15011edd9..9748458f1 100644
--- a/eth2/types/src/test_utils/bitfield.rs
+++ b/eth2/types/src/test_utils/test_random/bitfield.rs
@@ -1,5 +1,5 @@
-use super::super::Bitfield;
 use super::TestRandom;
+use crate::Bitfield;
 use rand::RngCore;
 
 impl<T: RngCore> TestRandom<T> for Bitfield {
diff --git a/eth2/types/src/test_utils/hash256.rs b/eth2/types/src/test_utils/test_random/hash256.rs
similarity index 100%
rename from eth2/types/src/test_utils/hash256.rs
rename to eth2/types/src/test_utils/test_random/hash256.rs
diff --git a/eth2/types/src/test_utils/public_key.rs b/eth2/types/src/test_utils/test_random/public_key.rs
similarity index 100%
rename from eth2/types/src/test_utils/public_key.rs
rename to eth2/types/src/test_utils/test_random/public_key.rs
diff --git a/eth2/types/src/test_utils/secret_key.rs b/eth2/types/src/test_utils/test_random/secret_key.rs
similarity index 100%
rename from eth2/types/src/test_utils/secret_key.rs
rename to eth2/types/src/test_utils/test_random/secret_key.rs
diff --git a/eth2/types/src/test_utils/signature.rs b/eth2/types/src/test_utils/test_random/signature.rs
similarity index 100%
rename from eth2/types/src/test_utils/signature.rs
rename to eth2/types/src/test_utils/test_random/signature.rs
diff --git a/eth2/types/src/test_utils/testing_attestation_builder.rs b/eth2/types/src/test_utils/testing_attestation_builder.rs
new file mode 100644
index 000000000..8c86d756d
--- /dev/null
+++ b/eth2/types/src/test_utils/testing_attestation_builder.rs
@@ -0,0 +1,126 @@
+use crate::*;
+use ssz::TreeHash;
+
+/// Builds an attestation to be used for testing purposes.
+///
+/// This struct should **never be used for production purposes.**
+pub struct TestingAttestationBuilder {
+    committee: Vec<usize>,
+    attestation: Attestation,
+}
+
+impl TestingAttestationBuilder {
+    /// Create a new attestation builder.
+    pub fn new(
+        state: &BeaconState,
+        committee: &[usize],
+        slot: Slot,
+        shard: u64,
+        spec: &ChainSpec,
+    ) -> Self {
+        let current_epoch = state.current_epoch(spec);
+        let previous_epoch = state.previous_epoch(spec);
+
+        let is_previous_epoch =
+            state.slot.epoch(spec.slots_per_epoch) != slot.epoch(spec.slots_per_epoch);
+
+        let justified_epoch = if is_previous_epoch {
+            state.previous_justified_epoch
+        } else {
+            state.justified_epoch
+        };
+
+        let epoch_boundary_root = if is_previous_epoch {
+            *state
+                .get_block_root(previous_epoch.start_slot(spec.slots_per_epoch), spec)
+                .unwrap()
+        } else {
+            *state
+                .get_block_root(current_epoch.start_slot(spec.slots_per_epoch), spec)
+                .unwrap()
+        };
+
+        let justified_block_root = *state
+            .get_block_root(justified_epoch.start_slot(spec.slots_per_epoch), spec)
+            .unwrap();
+
+        let mut aggregation_bitfield = Bitfield::new();
+        let mut custody_bitfield = Bitfield::new();
+
+        for (i, _) in committee.iter().enumerate() {
+            custody_bitfield.set(i, false);
+            aggregation_bitfield.set(i, false);
+        }
+
+        let attestation = Attestation {
+            aggregation_bitfield,
+            data: AttestationData {
+                slot,
+                shard,
+                beacon_block_root: *state.get_block_root(slot, spec).unwrap(),
+                epoch_boundary_root,
+                crosslink_data_root: Hash256::zero(),
+                latest_crosslink: state.latest_crosslinks[shard as usize].clone(),
+                justified_epoch,
+                justified_block_root,
+            },
+            custody_bitfield,
+            aggregate_signature: AggregateSignature::new(),
+        };
+
+        Self {
+            attestation,
+            committee: committee.to_vec(),
+        }
+    }
+
+    /// Signs the attestation with a subset (or all) committee members.
+    ///
+    /// `secret_keys` must be supplied in the same order as `signing_validators`. I.e., the first
+    /// keypair must be that of the first signing validator.
+    pub fn sign(
+        &mut self,
+        signing_validators: &[usize],
+        secret_keys: &[&SecretKey],
+        fork: &Fork,
+        spec: &ChainSpec,
+    ) {
+        assert_eq!(
+            signing_validators.len(),
+            secret_keys.len(),
+            "Must be a key for each validator"
+        );
+
+        for (key_index, validator_index) in signing_validators.iter().enumerate() {
+            let committee_index = self
+                .committee
+                .iter()
+                .position(|v| *v == *validator_index)
+                .expect("Signing validator not in attestation committee");
+
+            self.attestation
+                .aggregation_bitfield
+                .set(committee_index, true);
+
+            let message = AttestationDataAndCustodyBit {
+                data: self.attestation.data.clone(),
+                custody_bit: false,
+            }
+            .hash_tree_root();
+
+            let domain = spec.get_domain(
+                self.attestation.data.slot.epoch(spec.slots_per_epoch),
+                Domain::Attestation,
+                fork,
+            );
+
+            let signature = Signature::new(&message, domain, secret_keys[key_index]);
+            self.attestation.aggregate_signature.add(&signature)
+        }
+    }
+
+    /// Consume the builder and return the attestation.
+    pub fn build(self) -> Attestation {
+        self.attestation
+    }
+}
diff --git a/eth2/types/src/attester_slashing/builder.rs b/eth2/types/src/test_utils/testing_attester_slashing_builder.rs
similarity index 93%
rename from eth2/types/src/attester_slashing/builder.rs
rename to eth2/types/src/test_utils/testing_attester_slashing_builder.rs
index 05301f30b..232de87ec 100644
--- a/eth2/types/src/attester_slashing/builder.rs
+++ b/eth2/types/src/test_utils/testing_attester_slashing_builder.rs
@@ -2,9 +2,11 @@ use crate::*;
 use ssz::TreeHash;
 
 /// Builds an `AttesterSlashing`.
-pub struct AttesterSlashingBuilder();
+///
+/// This struct should **never be used for production purposes.**
+pub struct TestingAttesterSlashingBuilder();
 
-impl AttesterSlashingBuilder {
+impl TestingAttesterSlashingBuilder {
     /// Builds an `AttesterSlashing` that is a double vote.
     ///
     /// The `signer` function is used to sign the double-vote and accepts:
@@ -66,6 +68,7 @@ impl AttesterSlashingBuilder {
 
         let add_signatures = |attestation: &mut SlashableAttestation| {
             for (i, validator_index) in validator_indices.iter().enumerate() {
+                attestation.custody_bitfield.set(i, false);
                 let attestation_data_and_custody_bit = AttestationDataAndCustodyBit {
                     data: attestation.data.clone(),
                     custody_bit: attestation.custody_bitfield.get(i).unwrap(),
diff --git a/eth2/types/src/test_utils/testing_beacon_block_builder.rs b/eth2/types/src/test_utils/testing_beacon_block_builder.rs
new file mode 100644
index 000000000..97e395e1f
--- /dev/null
+++ b/eth2/types/src/test_utils/testing_beacon_block_builder.rs
@@ -0,0 +1,270 @@
+use crate::{
+    test_utils::{
+        TestingAttestationBuilder, TestingAttesterSlashingBuilder, TestingDepositBuilder,
+        TestingProposerSlashingBuilder, TestingTransferBuilder, TestingVoluntaryExitBuilder,
+    },
+    *,
+};
+use rayon::prelude::*;
+use ssz::{SignedRoot, TreeHash};
+
+/// Builds a beacon block to be used for testing purposes.
+///
+/// This struct should **never be used for production purposes.**
+pub struct TestingBeaconBlockBuilder {
+    block: BeaconBlock,
+}
+
+impl TestingBeaconBlockBuilder {
+    /// Create a new builder from genesis.
+    pub fn new(spec: &ChainSpec) -> Self {
+        Self {
+            block: BeaconBlock::genesis(spec.zero_hash, spec),
+        }
+    }
+
+    /// Set the slot of the block.
+    pub fn set_slot(&mut self, slot: Slot) {
+        self.block.slot = slot;
+    }
+
+    /// Signs the block.
+    ///
+    /// Modifying the block after signing may invalidate the signature.
+    pub fn sign(&mut self, sk: &SecretKey, fork: &Fork, spec: &ChainSpec) {
+        let proposal = self.block.proposal(spec);
+        let message = proposal.signed_root();
+        let epoch = self.block.slot.epoch(spec.slots_per_epoch);
+        let domain = spec.get_domain(epoch, Domain::Proposal, fork);
+        self.block.signature = Signature::new(&message, domain, sk);
+    }
+
+    /// Sets the randao to be a signature across the blocks epoch.
+    ///
+    /// Modifying the block's slot after signing may invalidate the signature.
+    pub fn set_randao_reveal(&mut self, sk: &SecretKey, fork: &Fork, spec: &ChainSpec) {
+        let epoch = self.block.slot.epoch(spec.slots_per_epoch);
+        let message = epoch.hash_tree_root();
+        let domain = spec.get_domain(epoch, Domain::Randao, fork);
+        self.block.randao_reveal = Signature::new(&message, domain, sk);
+    }
+
+    /// Inserts a signed, valid `ProposerSlashing` for the validator.
+    pub fn insert_proposer_slashing(
+        &mut self,
+        validator_index: u64,
+        secret_key: &SecretKey,
+        fork: &Fork,
+        spec: &ChainSpec,
+    ) {
+        let proposer_slashing = build_proposer_slashing(validator_index, secret_key, fork, spec);
+        self.block.body.proposer_slashings.push(proposer_slashing);
+    }
+
+    /// Inserts a signed, valid `AttesterSlashing` for each validator index in `validator_indices`.
+    pub fn insert_attester_slashing(
+        &mut self,
+        validator_indices: &[u64],
+        secret_keys: &[&SecretKey],
+        fork: &Fork,
+        spec: &ChainSpec,
+    ) {
+        let attester_slashing =
+            build_double_vote_attester_slashing(validator_indices, secret_keys, fork, spec);
+        self.block.body.attester_slashings.push(attester_slashing);
+    }
+
+    /// Fills the block with `MAX_ATTESTATIONS` attestations.
+    ///
+    /// It will first go and get each committee that is able to include an attestation in this
+    /// block. If there are enough committees, it will produce an attestation for each. If there
+    /// are _not_ enough committees, it will start splitting the committees in half until it
+    /// achieves the target. It will then produce separate attestations for each split committee.
+    ///
+    /// Note: the signed messages of the split committees will be identical -- it would be possible
+    /// to aggregate these split attestations.
+    pub fn fill_with_attestations(
+        &mut self,
+        state: &BeaconState,
+        secret_keys: &[&SecretKey],
+        spec: &ChainSpec,
+    ) -> Result<(), BeaconStateError> {
+        let mut slot = self.block.slot - spec.min_attestation_inclusion_delay;
+        let mut attestations_added = 0;
+
+        // Stores the following (in order):
+        //
+        // - The slot of the committee.
+        // - A list of all validators in the committee.
+        // - A list of all validators in the committee that should sign the attestation.
+        // - The shard of the committee.
+        let mut committees: Vec<(Slot, Vec<usize>, Vec<usize>, u64)> = vec![];
+
+        // Loop backwards through slots gathering each committee, until:
+        //
+        // - The slot is too old to be included in a block at this slot.
+        // - The `MAX_ATTESTATIONS`.
+        loop {
+            if state.slot >= slot + spec.slots_per_epoch {
+                break;
+            }
+
+            for (committee, shard) in state.get_crosslink_committees_at_slot(slot, spec)? {
+                if attestations_added >= spec.max_attestations {
+                    break;
+                }
+
+                committees.push((slot, committee.clone(), committee.clone(), *shard));
+
+                attestations_added += 1;
+            }
+
+            slot -= 1;
+        }
+
+        // Loop through all the committees, splitting each one in half until we have
+        // `MAX_ATTESTATIONS` committees.
+        loop {
+            if committees.len() >= spec.max_attestations as usize {
+                break;
+            }
+
+            for index in 0..committees.len() {
+                if committees.len() >= spec.max_attestations as usize {
+                    break;
+                }
+
+                let (slot, committee, mut signing_validators, shard) = committees[index].clone();
+
+                let new_signing_validators =
+                    signing_validators.split_off(signing_validators.len() / 2);
+
+                committees[index] = (slot, committee.clone(), signing_validators, shard);
+                committees.push((slot, committee, new_signing_validators, shard));
+            }
+        }
+
+        let mut attestations: Vec<Attestation> = committees
+            .par_iter()
+            .map(|(slot, committee, signing_validators, shard)| {
+                let mut builder =
+                    TestingAttestationBuilder::new(state, committee, *slot, *shard, spec);
+
+                let signing_secret_keys: Vec<&SecretKey> = signing_validators
+                    .iter()
+                    .map(|validator_index| secret_keys[*validator_index])
+                    .collect();
+                builder.sign(signing_validators, &signing_secret_keys, &state.fork, spec);
+
+                builder.build()
+            })
+            .collect();
+
+        self.block.body.attestations.append(&mut attestations);
+
+        Ok(())
+    }
+
+    /// Insert a `Valid` deposit into the state.
+    pub fn insert_deposit(
+        &mut self,
+        amount: u64,
+        index: u64,
+        state: &BeaconState,
+        spec: &ChainSpec,
+    ) {
+        let keypair = Keypair::random();
+
+        let mut builder = TestingDepositBuilder::new(amount);
+        builder.set_index(index);
+        builder.sign(&keypair, state, spec);
+
+        self.block.body.deposits.push(builder.build())
+    }
+
+    /// Insert a `Valid` exit into the state.
+    pub fn insert_exit(
+        &mut self,
+        state: &BeaconState,
+        validator_index: u64,
+        secret_key: &SecretKey,
+        spec: &ChainSpec,
+    ) {
+        let mut builder = TestingVoluntaryExitBuilder::new(
+            state.slot.epoch(spec.slots_per_epoch),
+            validator_index,
+        );
+
+        builder.sign(secret_key, &state.fork, spec);
+
+        self.block.body.voluntary_exits.push(builder.build())
+    }
+
+    /// Insert a `Valid` transfer into the state.
+    ///
+    /// Note: this will set the validator to be withdrawable by directly modifying the state
+    /// validator registry. This _may_ cause problems historic hashes, etc.
+    pub fn insert_transfer(
+        &mut self,
+        state: &BeaconState,
+        from: u64,
+        to: u64,
+        amount: u64,
+        keypair: Keypair,
+        spec: &ChainSpec,
+    ) {
+        let mut builder = TestingTransferBuilder::new(from, to, amount, state.slot);
+        builder.sign(keypair, &state.fork, spec);
+
+        self.block.body.transfers.push(builder.build())
+    }
+
+    /// Signs and returns the block, consuming the builder.
+    pub fn build(mut self, sk: &SecretKey, fork: &Fork, spec: &ChainSpec) -> BeaconBlock {
+        self.sign(sk, fork, spec);
+        self.block
+    }
+
+    /// Returns the block, consuming the builder.
+    pub fn build_without_signing(self) -> BeaconBlock {
+        self.block
+    }
+}
+
+/// Builds an `ProposerSlashing` for some `validator_index`.
+///
+/// Signs the message using a `BeaconChainHarness`.
+fn build_proposer_slashing(
+    validator_index: u64,
+    secret_key: &SecretKey,
+    fork: &Fork,
+    spec: &ChainSpec,
+) -> ProposerSlashing {
+    let signer = |_validator_index: u64, message: &[u8], epoch: Epoch, domain: Domain| {
+        let domain = spec.get_domain(epoch, domain, fork);
+        Signature::new(message, domain, secret_key)
+    };
+
+    TestingProposerSlashingBuilder::double_vote(validator_index, signer, spec)
+}
+
+/// Builds an `AttesterSlashing` for some `validator_indices`.
+///
+/// Signs the message using a `BeaconChainHarness`.
+fn build_double_vote_attester_slashing(
+    validator_indices: &[u64],
+    secret_keys: &[&SecretKey],
+    fork: &Fork,
+    spec: &ChainSpec,
+) -> AttesterSlashing {
+    let signer = |validator_index: u64, message: &[u8], epoch: Epoch, domain: Domain| {
+        let key_index = validator_indices
+            .iter()
+            .position(|&i| i == validator_index)
+            .expect("Unable to find attester slashing key");
+        let domain = spec.get_domain(epoch, domain, fork);
+        Signature::new(message, domain, secret_keys[key_index])
+    };
+
+    TestingAttesterSlashingBuilder::double_vote(validator_indices, signer)
+}
diff --git a/eth2/types/src/test_utils/testing_beacon_state_builder.rs b/eth2/types/src/test_utils/testing_beacon_state_builder.rs
new file mode 100644
index 000000000..b2cf28c8a
--- /dev/null
+++ b/eth2/types/src/test_utils/testing_beacon_state_builder.rs
@@ -0,0 +1,284 @@
+use super::{generate_deterministic_keypairs, KeypairsFile};
+use crate::beacon_state::BeaconStateBuilder;
+use crate::*;
+use bls::get_withdrawal_credentials;
+use dirs;
+use log::debug;
+use rayon::prelude::*;
+use std::path::{Path, PathBuf};
+
+pub const KEYPAIRS_FILE: &str = "keypairs.raw_keypairs";
+
+/// Returns the directory where the generated keypairs should be stored.
+///
+/// It is either `$HOME/.lighthouse/keypairs.raw_keypairs` or, if `$HOME` is not available,
+/// `./keypairs.raw_keypairs`.
+pub fn keypairs_path() -> PathBuf {
+    let dir = dirs::home_dir()
+        .and_then(|home| Some(home.join(".lighthouse")))
+        .unwrap_or_else(|| PathBuf::from(""));
+    dir.join(KEYPAIRS_FILE)
+}
+
+/// Builds a beacon state to be used for testing purposes.
+///
+/// This struct should **never be used for production purposes.**
+pub struct TestingBeaconStateBuilder {
+    state: BeaconState,
+    keypairs: Vec<Keypair>,
+}
+
+impl TestingBeaconStateBuilder {
+    /// Attempts to load validators from a file in `$HOME/.lighthouse/keypairs.raw_keypairs`. If
+    /// the file is unavailable, it generates the keys at runtime.
+    ///
+    /// If the `$HOME` environment variable is not set, the local directory is used.
+    ///
+    /// See the `Self::from_keypairs_file` method for more info.
+    ///
+    /// # Panics
+    ///
+    /// If the file does not contain enough keypairs or is invalid.
+    pub fn from_default_keypairs_file_if_exists(validator_count: usize, spec: &ChainSpec) -> Self {
+        let dir = dirs::home_dir()
+            .and_then(|home| Some(home.join(".lighthouse")))
+            .unwrap_or_else(|| PathBuf::from(""));
+        let file = dir.join(KEYPAIRS_FILE);
+
+        if file.exists() {
+            TestingBeaconStateBuilder::from_keypairs_file(validator_count, &file, spec)
+        } else {
+            TestingBeaconStateBuilder::from_deterministic_keypairs(validator_count, spec)
+        }
+    }
+
+    /// Loads the initial validator keypairs from a file on disk.
+    ///
+    /// Loading keypairs from file is ~10x faster than generating them. Use the `gen_keys` command
+    /// on the  `test_harness` binary to generate the keys. In the `test_harness` dir, run `cargo
+    /// run -- gen_keys -h` for help.
+    ///
+    /// # Panics
+    ///
+    /// If the file does not exist, is invalid or does not contain enough keypairs.
+    pub fn from_keypairs_file(validator_count: usize, path: &Path, spec: &ChainSpec) -> Self {
+        debug!("Loading {} keypairs from file...", validator_count);
+        let keypairs = Vec::from_raw_file(path, validator_count).unwrap();
+        TestingBeaconStateBuilder::from_keypairs(keypairs, spec)
+    }
+
+    /// Generates the validator keypairs deterministically.
+    pub fn from_deterministic_keypairs(validator_count: usize, spec: &ChainSpec) -> Self {
+        debug!("Generating {} deterministic keypairs...", validator_count);
+        let keypairs = generate_deterministic_keypairs(validator_count);
+        TestingBeaconStateBuilder::from_keypairs(keypairs, spec)
+    }
+
+    /// Creates the builder from an existing set of keypairs.
+    pub fn from_keypairs(keypairs: Vec<Keypair>, spec: &ChainSpec) -> Self {
+        let validator_count = keypairs.len();
+
+        debug!(
+            "Building {} Validator objects from keypairs...",
+            validator_count
+        );
+        let validators = keypairs
+            .par_iter()
+            .map(|keypair| {
+                let withdrawal_credentials = Hash256::from_slice(&get_withdrawal_credentials(
+                    &keypair.pk,
+                    spec.bls_withdrawal_prefix_byte,
+                ));
+
+                Validator {
+                    pubkey: keypair.pk.clone(),
+                    withdrawal_credentials,
+                    activation_epoch: spec.far_future_epoch,
+                    exit_epoch: spec.far_future_epoch,
+                    withdrawable_epoch: spec.far_future_epoch,
+                    initiated_exit: false,
+                    slashed: false,
+                }
+            })
+            .collect();
+
+        let mut state_builder = BeaconStateBuilder::new(
+            0,
+            Eth1Data {
+                deposit_root: Hash256::zero(),
+                block_hash: Hash256::zero(),
+            },
+            spec,
+        );
+
+        let balances = vec![32_000_000_000; validator_count];
+
+        debug!("Importing {} existing validators...", validator_count);
+        state_builder.import_existing_validators(
+            validators,
+            balances,
+            validator_count as u64,
+            spec,
+        );
+
+        let state = state_builder.build(spec).unwrap();
+
+        debug!("BeaconState built.");
+
+        Self { state, keypairs }
+    }
+
+    /// Consume the builder and return the `BeaconState` and the keypairs for each validator.
+    pub fn build(self) -> (BeaconState, Vec<Keypair>) {
+        (self.state, self.keypairs)
+    }
+
+    /// Ensures that the state returned from `Self::build(..)` has all caches pre-built.
+    ///
+    /// Note: this performs the build when called. Ensure that no changes are made that would
+    /// invalidate this cache.
+    pub fn build_caches(&mut self, spec: &ChainSpec) -> Result<(), BeaconStateError> {
+        let state = &mut self.state;
+
+        state.build_epoch_cache(RelativeEpoch::Previous, &spec)?;
+        state.build_epoch_cache(RelativeEpoch::Current, &spec)?;
+        state.build_epoch_cache(RelativeEpoch::Next, &spec)?;
+
+        Ok(())
+    }
+
+    /// Sets the `BeaconState` to be in a slot, calling `teleport_to_epoch` to update the epoch.
+    pub fn teleport_to_slot(&mut self, slot: Slot, spec: &ChainSpec) {
+        self.teleport_to_epoch(slot.epoch(spec.slots_per_epoch), spec);
+        self.state.slot = slot;
+    }
+
+    /// Sets the `BeaconState` to be in the first slot of the given epoch.
+    ///
+    /// Sets all justification/finalization parameters to be be as "perfect" as possible (i.e.,
+    /// highest justified and finalized slots, full justification bitfield, etc).
+    fn teleport_to_epoch(&mut self, epoch: Epoch, spec: &ChainSpec) {
+        let state = &mut self.state;
+
+        let slot = epoch.start_slot(spec.slots_per_epoch);
+
+        state.slot = slot;
+
+        state.previous_shuffling_epoch = epoch - 1;
+        state.current_shuffling_epoch = epoch;
+
+        state.previous_shuffling_seed = Hash256::from_low_u64_le(0);
+        state.current_shuffling_seed = Hash256::from_low_u64_le(1);
+
+        state.previous_justified_epoch = epoch - 3;
+        state.justified_epoch = epoch - 2;
+        state.justification_bitfield = u64::max_value();
+
+        state.finalized_epoch = epoch - 3;
+        state.validator_registry_update_epoch = epoch - 3;
+    }
+
+    /// Creates a full set of attestations for the `BeaconState`. Each attestation has full
+    /// participation from its committee and references the expected beacon_block hashes.
+    ///
+    /// These attestations should be fully conducive to justification and finalization.
+    pub fn insert_attestations(&mut self, spec: &ChainSpec) {
+        let state = &mut self.state;
+
+        state
+            .build_epoch_cache(RelativeEpoch::Previous, spec)
+            .unwrap();
+        state
+            .build_epoch_cache(RelativeEpoch::Current, spec)
+            .unwrap();
+
+        let current_epoch = state.current_epoch(spec);
+        let previous_epoch = state.previous_epoch(spec);
+
+        let first_slot = previous_epoch.start_slot(spec.slots_per_epoch).as_u64();
+        let last_slot = current_epoch.end_slot(spec.slots_per_epoch).as_u64()
+            - spec.min_attestation_inclusion_delay;
+        let last_slot = std::cmp::min(state.slot.as_u64(), last_slot);
+
+        for slot in first_slot..last_slot + 1 {
+            let slot = Slot::from(slot);
+
+            let committees = state
+                .get_crosslink_committees_at_slot(slot, spec)
+                .unwrap()
+                .clone();
+
+            for (committee, shard) in committees {
+                state
+                    .latest_attestations
+                    .push(committee_to_pending_attestation(
+                        state, &committee, shard, slot, spec,
+                    ))
+            }
+        }
+    }
+}
+
+/// Maps a committee to a `PendingAttestation`.
+///
+/// The committee will be signed by all validators in the committee.
+fn committee_to_pending_attestation(
+    state: &BeaconState,
+    committee: &[usize],
+    shard: u64,
+    slot: Slot,
+    spec: &ChainSpec,
+) -> PendingAttestation {
+    let current_epoch = state.current_epoch(spec);
+    let previous_epoch = state.previous_epoch(spec);
+
+    let mut aggregation_bitfield = Bitfield::new();
+    let mut custody_bitfield = Bitfield::new();
+
+    for (i, _) in committee.iter().enumerate() {
+        aggregation_bitfield.set(i, true);
+        custody_bitfield.set(i, true);
+    }
+
+    let is_previous_epoch =
+        state.slot.epoch(spec.slots_per_epoch) != slot.epoch(spec.slots_per_epoch);
+
+    let justified_epoch = if is_previous_epoch {
+        state.previous_justified_epoch
+    } else {
+        state.justified_epoch
+    };
+
+    let epoch_boundary_root = if is_previous_epoch {
+        *state
+            .get_block_root(previous_epoch.start_slot(spec.slots_per_epoch), spec)
+            .unwrap()
+    } else {
+        *state
+            .get_block_root(current_epoch.start_slot(spec.slots_per_epoch), spec)
+            .unwrap()
+    };
+
+    let justified_block_root = *state
+        .get_block_root(justified_epoch.start_slot(spec.slots_per_epoch), spec)
+        .unwrap();
+
+    PendingAttestation {
+        aggregation_bitfield,
+        data: AttestationData {
+            slot,
+            shard,
+            beacon_block_root: *state.get_block_root(slot, spec).unwrap(),
+            epoch_boundary_root,
+            crosslink_data_root: Hash256::zero(),
+            latest_crosslink: Crosslink {
+                epoch: slot.epoch(spec.slots_per_epoch),
+                crosslink_data_root: Hash256::zero(),
+            },
+            justified_epoch,
+            justified_block_root,
+        },
+        custody_bitfield,
+        inclusion_slot: slot + spec.min_attestation_inclusion_delay,
+    }
+}
diff --git a/eth2/types/src/test_utils/testing_deposit_builder.rs b/eth2/types/src/test_utils/testing_deposit_builder.rs
new file mode 100644
index 000000000..729311468
--- /dev/null
+++ b/eth2/types/src/test_utils/testing_deposit_builder.rs
@@ -0,0 +1,64 @@
+use crate::*;
+use bls::get_withdrawal_credentials;
+
+/// Builds an deposit to be used for testing purposes.
+///
+/// This struct should **never be used for production purposes.**
+pub struct TestingDepositBuilder {
+    deposit: Deposit,
+}
+
+impl TestingDepositBuilder {
+    /// Instantiates a new builder.
+    pub fn new(amount: u64) -> Self {
+        let keypair = Keypair::random();
+
+        let deposit = Deposit {
+            branch: vec![],
+            index: 0,
+            deposit_data: DepositData {
+                amount,
+                timestamp: 1,
+                deposit_input: DepositInput {
+                    pubkey: keypair.pk,
+                    withdrawal_credentials: Hash256::zero(),
+                    proof_of_possession: Signature::empty_signature(),
+                },
+            },
+        };
+
+        Self { deposit }
+    }
+
+    /// Set the `deposit.index` value.
+    pub fn set_index(&mut self, index: u64) {
+        self.deposit.index = index;
+    }
+
+    /// Signs the deposit, also setting the following values:
+    ///
+    /// - `pubkey` to the signing pubkey.
+    /// - `withdrawal_credentials` to the signing pubkey.
+    /// - `proof_of_possesssion`
+    pub fn sign(&mut self, keypair: &Keypair, state: &BeaconState, spec: &ChainSpec) {
+        let withdrawal_credentials = Hash256::from_slice(
+            &get_withdrawal_credentials(&keypair.pk, spec.bls_withdrawal_prefix_byte)[..],
+        );
+
+        let epoch = state.current_epoch(spec);
+        let domain = spec.get_domain(epoch, Domain::Deposit, &state.fork);
+
+        self.deposit.deposit_data.deposit_input.pubkey = keypair.pk.clone();
+        self.deposit
+            .deposit_data
+            .deposit_input
+            .withdrawal_credentials = withdrawal_credentials.clone();
+        self.deposit.deposit_data.deposit_input.proof_of_possession =
+            DepositInput::create_proof_of_possession(&keypair, &withdrawal_credentials, domain);
+    }
+
+    /// Builds the deposit, consuming the builder.
+    pub fn build(self) -> Deposit {
+        self.deposit
+    }
+}
diff --git a/eth2/types/src/proposer_slashing/builder.rs b/eth2/types/src/test_utils/testing_proposer_slashing_builder.rs
similarity index 91%
rename from eth2/types/src/proposer_slashing/builder.rs
rename to eth2/types/src/test_utils/testing_proposer_slashing_builder.rs
index 472a76ec1..7f16b679f 100644
--- a/eth2/types/src/proposer_slashing/builder.rs
+++ b/eth2/types/src/test_utils/testing_proposer_slashing_builder.rs
@@ -2,9 +2,11 @@ use crate::*;
 use ssz::SignedRoot;
 
 /// Builds a `ProposerSlashing`.
-pub struct ProposerSlashingBuilder();
+///
+/// This struct should **never be used for production purposes.**
+pub struct TestingProposerSlashingBuilder();
 
-impl ProposerSlashingBuilder {
+impl TestingProposerSlashingBuilder {
     /// Builds a `ProposerSlashing` that is a double vote.
     ///
     /// The `signer` function is used to sign the double-vote and accepts:
diff --git a/eth2/types/src/test_utils/testing_transfer_builder.rs b/eth2/types/src/test_utils/testing_transfer_builder.rs
new file mode 100644
index 000000000..c4256ebea
--- /dev/null
+++ b/eth2/types/src/test_utils/testing_transfer_builder.rs
@@ -0,0 +1,45 @@
+use crate::*;
+use ssz::SignedRoot;
+
+/// Builds a transfer to be used for testing purposes.
+///
+/// This struct should **never be used for production purposes.**
+pub struct TestingTransferBuilder {
+    transfer: Transfer,
+}
+
+impl TestingTransferBuilder {
+    /// Instantiates a new builder.
+    pub fn new(from: u64, to: u64, amount: u64, slot: Slot) -> Self {
+        let keypair = Keypair::random();
+
+        let transfer = Transfer {
+            from,
+            to,
+            amount,
+            fee: 0,
+            slot,
+            pubkey: keypair.pk,
+            signature: Signature::empty_signature(),
+        };
+
+        Self { transfer }
+    }
+
+    /// Signs the transfer.
+    ///
+    /// The keypair must match that of the `from` validator index.
+    pub fn sign(&mut self, keypair: Keypair, fork: &Fork, spec: &ChainSpec) {
+        self.transfer.pubkey = keypair.pk;
+        let message = self.transfer.signed_root();
+        let epoch = self.transfer.slot.epoch(spec.slots_per_epoch);
+        let domain = spec.get_domain(epoch, Domain::Transfer, fork);
+
+        self.transfer.signature = Signature::new(&message, domain, &keypair.sk);
+    }
+
+    /// Builds the transfer, consuming the builder.
+    pub fn build(self) -> Transfer {
+        self.transfer
+    }
+}
diff --git a/eth2/types/src/test_utils/testing_voluntary_exit_builder.rs b/eth2/types/src/test_utils/testing_voluntary_exit_builder.rs
new file mode 100644
index 000000000..fe5c8325a
--- /dev/null
+++ b/eth2/types/src/test_utils/testing_voluntary_exit_builder.rs
@@ -0,0 +1,37 @@
+use crate::*;
+use ssz::SignedRoot;
+
+/// Builds an exit to be used for testing purposes.
+///
+/// This struct should **never be used for production purposes.**
+pub struct TestingVoluntaryExitBuilder {
+    exit: VoluntaryExit,
+}
+
+impl TestingVoluntaryExitBuilder {
+    /// Instantiates a new builder.
+    pub fn new(epoch: Epoch, validator_index: u64) -> Self {
+        let exit = VoluntaryExit {
+            epoch,
+            validator_index,
+            signature: Signature::empty_signature(),
+        };
+
+        Self { exit }
+    }
+
+    /// Signs the exit.
+    ///
+    /// The signing secret key must match that of the exiting validator.
+    pub fn sign(&mut self, secret_key: &SecretKey, fork: &Fork, spec: &ChainSpec) {
+        let message = self.exit.signed_root();
+        let domain = spec.get_domain(self.exit.epoch, Domain::Exit, fork);
+
+        self.exit.signature = Signature::new(&message, domain, secret_key);
+    }
+
+    /// Builds the exit, consuming the builder.
+    pub fn build(self) -> VoluntaryExit {
+        self.exit
+    }
+}
diff --git a/eth2/types/src/validator.rs b/eth2/types/src/validator.rs
index 43701ca05..59f6c5826 100644
--- a/eth2/types/src/validator.rs
+++ b/eth2/types/src/validator.rs
@@ -1,13 +1,13 @@
 use crate::{test_utils::TestRandom, Epoch, Hash256, PublicKey};
 use rand::RngCore;
-use serde_derive::Serialize;
+use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode, TreeHash};
 use test_random_derive::TestRandom;
 
 /// Information about a `BeaconChain` validator.
 ///
 /// Spec v0.4.0
-#[derive(Debug, Clone, PartialEq, Serialize, Encode, Decode, TestRandom, TreeHash)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize, Encode, Decode, TestRandom, TreeHash)]
 pub struct Validator {
     pub pubkey: PublicKey,
     pub withdrawal_credentials: Hash256,
diff --git a/eth2/utils/bls/Cargo.toml b/eth2/utils/bls/Cargo.toml
index 7a436307b..468ed8050 100644
--- a/eth2/utils/bls/Cargo.toml
+++ b/eth2/utils/bls/Cargo.toml
@@ -5,8 +5,9 @@ authors = ["Paul Hauner <paul@paulhauner.com>"]
 edition = "2018"
 
 [dependencies]
-bls-aggregates = { git = "https://github.com/sigp/signature-schemes", tag = "0.5.2" }
+bls-aggregates = { git = "https://github.com/sigp/signature-schemes", tag = "0.6.0" }
 hashing = { path = "../hashing" }
 hex = "0.3"
 serde = "1.0"
+serde_derive = "1.0"
 ssz = { path = "../ssz" }
diff --git a/eth2/utils/bls/src/aggregate_signature.rs b/eth2/utils/bls/src/aggregate_signature.rs
index 2d8776353..af0879ec7 100644
--- a/eth2/utils/bls/src/aggregate_signature.rs
+++ b/eth2/utils/bls/src/aggregate_signature.rs
@@ -48,16 +48,8 @@ impl AggregateSignature {
         domain: u64,
         aggregate_public_keys: &[&AggregatePublicKey],
     ) -> bool {
-        // TODO: the API for `RawAggregatePublicKey` shoudn't need to take an owned
-        // `AggregatePublicKey`. There is an issue to fix this, but in the meantime we need to
-        // clone.
-        //
-        // https://github.com/sigp/signature-schemes/issues/10
-        let aggregate_public_keys: Vec<RawAggregatePublicKey> = aggregate_public_keys
-            .iter()
-            .map(|pk| pk.as_raw())
-            .cloned()
-            .collect();
+        let aggregate_public_keys: Vec<&RawAggregatePublicKey> =
+            aggregate_public_keys.iter().map(|pk| pk.as_raw()).collect();
 
         // Messages are concatenated into one long message.
         let mut msg: Vec<u8> = vec![];
diff --git a/eth2/utils/bls/src/keypair.rs b/eth2/utils/bls/src/keypair.rs
index 1cce9c10e..d60a2fc25 100644
--- a/eth2/utils/bls/src/keypair.rs
+++ b/eth2/utils/bls/src/keypair.rs
@@ -1,6 +1,7 @@
 use super::{PublicKey, SecretKey};
+use serde_derive::{Deserialize, Serialize};
 
-#[derive(Debug, Clone, PartialEq, Eq)]
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub struct Keypair {
     pub sk: SecretKey,
     pub pk: PublicKey,
diff --git a/eth2/utils/bls/src/lib.rs b/eth2/utils/bls/src/lib.rs
index bb109b0a1..38a129908 100644
--- a/eth2/utils/bls/src/lib.rs
+++ b/eth2/utils/bls/src/lib.rs
@@ -6,6 +6,7 @@ mod aggregate_signature;
 mod keypair;
 mod public_key;
 mod secret_key;
+mod serde_vistors;
 mod signature;
 
 pub use crate::aggregate_public_key::AggregatePublicKey;
@@ -20,20 +21,6 @@ pub const BLS_AGG_SIG_BYTE_SIZE: usize = 96;
 use hashing::hash;
 use ssz::ssz_encode;
 
-/// For some signature and public key, ensure that the signature message was the public key and it
-/// was signed by the secret key that corresponds to that public key.
-pub fn verify_proof_of_possession(sig: &Signature, pubkey: &PublicKey) -> bool {
-    // TODO: replace this function with state.validate_proof_of_possession
-    // https://github.com/sigp/lighthouse/issues/239
-    sig.verify(&ssz_encode(pubkey), 0, &pubkey)
-}
-
-// TODO: Update this method
-// https://github.com/sigp/lighthouse/issues/239
-pub fn create_proof_of_possession(keypair: &Keypair) -> Signature {
-    Signature::new(&ssz_encode(&keypair.pk), 0, &keypair.sk)
-}
-
 /// Returns the withdrawal credentials for a given public key.
 pub fn get_withdrawal_credentials(pubkey: &PublicKey, prefix_byte: u8) -> Vec<u8> {
     let hashed = hash(&ssz_encode(pubkey));
diff --git a/eth2/utils/bls/src/public_key.rs b/eth2/utils/bls/src/public_key.rs
index c7fd526a0..777ccceaa 100644
--- a/eth2/utils/bls/src/public_key.rs
+++ b/eth2/utils/bls/src/public_key.rs
@@ -1,6 +1,8 @@
+use super::serde_vistors::HexVisitor;
 use super::SecretKey;
 use bls_aggregates::PublicKey as RawPublicKey;
 use hex::encode as hex_encode;
+use serde::de::{Deserialize, Deserializer};
 use serde::ser::{Serialize, Serializer};
 use ssz::{
     decode_ssz_list, hash, ssz_encode, Decodable, DecodeError, Encodable, SszStream, TreeHash,
@@ -25,6 +27,24 @@ impl PublicKey {
         &self.0
     }
 
+    /// Converts compressed bytes to PublicKey
+    pub fn from_bytes(bytes: &[u8]) -> Result<Self, DecodeError> {
+        let pubkey = RawPublicKey::from_bytes(&bytes).map_err(|_| DecodeError::Invalid)?;
+        Ok(PublicKey(pubkey))
+    }
+
+    /// Returns the PublicKey as (x, y) bytes
+    pub fn as_uncompressed_bytes(&self) -> Vec<u8> {
+        RawPublicKey::as_uncompressed_bytes(&mut self.0.clone())
+    }
+
+    /// Converts (x, y) bytes to PublicKey
+    pub fn from_uncompressed_bytes(bytes: &[u8]) -> Result<Self, DecodeError> {
+        let pubkey =
+            RawPublicKey::from_uncompressed_bytes(&bytes).map_err(|_| DecodeError::Invalid)?;
+        Ok(PublicKey(pubkey))
+    }
+
     /// Returns the last 6 bytes of the SSZ encoding of the public key, as a hex string.
     ///
     /// Useful for providing a short identifier to the user.
@@ -61,7 +81,19 @@ impl Serialize for PublicKey {
     where
         S: Serializer,
     {
-        serializer.serialize_bytes(&ssz_encode(self))
+        serializer.serialize_str(&hex_encode(ssz_encode(self)))
+    }
+}
+
+impl<'de> Deserialize<'de> for PublicKey {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        let bytes = deserializer.deserialize_str(HexVisitor)?;
+        let (pubkey, _) = <_>::ssz_decode(&bytes[..], 0)
+            .map_err(|e| serde::de::Error::custom(format!("invalid ssz ({:?})", e)))?;
+        Ok(pubkey)
     }
 }
 
@@ -78,8 +110,14 @@ impl PartialEq for PublicKey {
 }
 
 impl Hash for PublicKey {
+    /// Note: this is distinct from consensus serialization, it will produce a different hash.
+    ///
+    /// This method uses the uncompressed bytes, which are much faster to obtain than the
+    /// compressed bytes required for consensus serialization.
+    ///
+    /// Use `ssz::Encode` to obtain the bytes required for consensus hashing.
     fn hash<H: Hasher>(&self, state: &mut H) {
-        ssz_encode(self).hash(state)
+        self.as_uncompressed_bytes().hash(state)
     }
 }
 
diff --git a/eth2/utils/bls/src/secret_key.rs b/eth2/utils/bls/src/secret_key.rs
index f2d54f4ac..06c968389 100644
--- a/eth2/utils/bls/src/secret_key.rs
+++ b/eth2/utils/bls/src/secret_key.rs
@@ -1,5 +1,9 @@
+use super::serde_vistors::HexVisitor;
 use bls_aggregates::{DecodeError as BlsDecodeError, SecretKey as RawSecretKey};
-use ssz::{decode_ssz_list, Decodable, DecodeError, Encodable, SszStream, TreeHash};
+use hex::encode as hex_encode;
+use serde::de::{Deserialize, Deserializer};
+use serde::ser::{Serialize, Serializer};
+use ssz::{decode_ssz_list, ssz_encode, Decodable, DecodeError, Encodable, SszStream, TreeHash};
 
 /// A single BLS signature.
 ///
@@ -40,6 +44,27 @@ impl Decodable for SecretKey {
     }
 }
 
+impl Serialize for SecretKey {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        serializer.serialize_str(&hex_encode(ssz_encode(self)))
+    }
+}
+
+impl<'de> Deserialize<'de> for SecretKey {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        let bytes = deserializer.deserialize_str(HexVisitor)?;
+        let (pubkey, _) = <_>::ssz_decode(&bytes[..], 0)
+            .map_err(|e| serde::de::Error::custom(format!("invalid ssz ({:?})", e)))?;
+        Ok(pubkey)
+    }
+}
+
 impl TreeHash for SecretKey {
     fn hash_tree_root_internal(&self) -> Vec<u8> {
         self.0.as_bytes().clone()
diff --git a/eth2/utils/bls/src/serde_vistors.rs b/eth2/utils/bls/src/serde_vistors.rs
new file mode 100644
index 000000000..55eadb883
--- /dev/null
+++ b/eth2/utils/bls/src/serde_vistors.rs
@@ -0,0 +1,20 @@
+use hex;
+use serde::de::{self, Visitor};
+use std::fmt;
+
+pub struct HexVisitor;
+
+impl<'de> Visitor<'de> for HexVisitor {
+    type Value = Vec<u8>;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("a hex string (without 0x prefix)")
+    }
+
+    fn visit_str<E>(self, value: &str) -> Result<Self::Value, E>
+    where
+        E: de::Error,
+    {
+        Ok(hex::decode(value).map_err(|e| de::Error::custom(format!("invalid hex ({:?})", e)))?)
+    }
+}
diff --git a/eth2/utils/bls/src/signature.rs b/eth2/utils/bls/src/signature.rs
index c0c31ef27..760b0018a 100644
--- a/eth2/utils/bls/src/signature.rs
+++ b/eth2/utils/bls/src/signature.rs
@@ -1,5 +1,8 @@
+use super::serde_vistors::HexVisitor;
 use super::{PublicKey, SecretKey};
 use bls_aggregates::Signature as RawSignature;
+use hex::encode as hex_encode;
+use serde::de::{Deserialize, Deserializer};
 use serde::ser::{Serialize, Serializer};
 use ssz::{
     decode_ssz_list, hash, ssz_encode, Decodable, DecodeError, Encodable, SszStream, TreeHash,
@@ -51,9 +54,12 @@ impl Signature {
 
     /// Returns a new empty signature.
     pub fn empty_signature() -> Self {
+        // Empty Signature is currently being represented as BLS::Signature.point_at_infinity()
+        // However it should be represented as vec![0; 96] but this
+        // would require all signatures to be represented in byte form as opposed to Signature
         let mut empty: Vec<u8> = vec![0; 96];
-        // TODO: Modify the way flags are used (b_flag should not be used for empty_signature in the future)
-        empty[0] += u8::pow(2, 6);
+        // Sets C_flag and B_flag to 1 and all else to 0
+        empty[0] += u8::pow(2, 6) + u8::pow(2, 7);
         Signature(RawSignature::from_bytes(&empty).unwrap())
     }
 }
@@ -83,7 +89,19 @@ impl Serialize for Signature {
     where
         S: Serializer,
     {
-        serializer.serialize_bytes(&ssz_encode(self))
+        serializer.serialize_str(&hex_encode(ssz_encode(self)))
+    }
+}
+
+impl<'de> Deserialize<'de> for Signature {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        let bytes = deserializer.deserialize_str(HexVisitor)?;
+        let (pubkey, _) = <_>::ssz_decode(&bytes[..], 0)
+            .map_err(|e| serde::de::Error::custom(format!("invalid ssz ({:?})", e)))?;
+        Ok(pubkey)
     }
 }
 
@@ -114,7 +132,7 @@ mod tests {
         assert_eq!(sig_as_bytes.len(), 96);
         for (i, one_byte) in sig_as_bytes.iter().enumerate() {
             if i == 0 {
-                assert_eq!(*one_byte, u8::pow(2, 6));
+                assert_eq!(*one_byte, u8::pow(2, 6) + u8::pow(2, 7));
             } else {
                 assert_eq!(*one_byte, 0);
             }