diff --git a/Cargo.toml b/Cargo.toml
index 42d69489b..c5aae7f43 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -9,6 +9,7 @@ members = [
 	"eth2/utils/boolean-bitfield",
 	"eth2/utils/hashing",
 	"eth2/utils/honey-badger-split",
+	"eth2/utils/merkle_proof",
 	"eth2/utils/int_to_bytes",
 	"eth2/utils/slot_clock",
 	"eth2/utils/ssz",
diff --git a/beacon_node/beacon_chain/src/beacon_chain.rs b/beacon_node/beacon_chain/src/beacon_chain.rs
index e33912f7a..ddb9514db 100644
--- a/beacon_node/beacon_chain/src/beacon_chain.rs
+++ b/beacon_node/beacon_chain/src/beacon_chain.rs
@@ -65,6 +65,7 @@ pub struct BeaconChain<T: ClientDB + Sized, U: SlotClock, F: ForkChoice> {
     pub slot_clock: U,
     pub attestation_aggregator: RwLock<AttestationAggregator>,
     pub deposits_for_inclusion: RwLock<Vec<Deposit>>,
+    pub exits_for_inclusion: RwLock<Vec<Exit>>,
     pub proposer_slashings_for_inclusion: RwLock<Vec<ProposerSlashing>>,
     pub attester_slashings_for_inclusion: RwLock<Vec<AttesterSlashing>>,
     canonical_head: RwLock<CheckPoint>,
@@ -134,6 +135,7 @@ where
             slot_clock,
             attestation_aggregator,
             deposits_for_inclusion: RwLock::new(vec![]),
+            exits_for_inclusion: RwLock::new(vec![]),
             proposer_slashings_for_inclusion: RwLock::new(vec![]),
             attester_slashings_for_inclusion: RwLock::new(vec![]),
             state: RwLock::new(genesis_state),
@@ -370,13 +372,17 @@ where
 
     /// Accept some deposit and queue it for inclusion in an appropriate block.
     pub fn receive_deposit_for_inclusion(&self, deposit: Deposit) {
-        // TODO: deposits are not check for validity; check them.
+        // TODO: deposits are not checked for validity; check them.
+        //
+        // https://github.com/sigp/lighthouse/issues/276
         self.deposits_for_inclusion.write().push(deposit);
     }
 
     /// Return a vec of deposits suitable for inclusion in some block.
     pub fn get_deposits_for_block(&self) -> Vec<Deposit> {
         // TODO: deposits are indiscriminately included; check them for validity.
+        //
+        // https://github.com/sigp/lighthouse/issues/275
         self.deposits_for_inclusion.read().clone()
     }
 
@@ -386,6 +392,8 @@ where
     /// This ensures that `Deposits` are not included twice in successive blocks.
     pub fn set_deposits_as_included(&self, included_deposits: &[Deposit]) {
         // TODO: method does not take forks into account; consider this.
+        //
+        // https://github.com/sigp/lighthouse/issues/275
         let mut indices_to_delete = vec![];
 
         for included in included_deposits {
@@ -402,9 +410,49 @@ where
         }
     }
 
+    /// Accept some exit and queue it for inclusion in an appropriate block.
+    pub fn receive_exit_for_inclusion(&self, exit: Exit) {
+        // TODO: exits are not checked for validity; check them.
+        //
+        // https://github.com/sigp/lighthouse/issues/276
+        self.exits_for_inclusion.write().push(exit);
+    }
+
+    /// Return a vec of exits suitable for inclusion in some block.
+    pub fn get_exits_for_block(&self) -> Vec<Exit> {
+        // TODO: exits are indiscriminately included; check them for validity.
+        //
+        // https://github.com/sigp/lighthouse/issues/275
+        self.exits_for_inclusion.read().clone()
+    }
+
+    /// Takes a list of `Deposits` that were included in recent blocks and removes them from the
+    /// inclusion queue.
+    ///
+    /// This ensures that `Deposits` are not included twice in successive blocks.
+    pub fn set_exits_as_included(&self, included_exits: &[Exit]) {
+        // TODO: method does not take forks into account; consider this.
+        let mut indices_to_delete = vec![];
+
+        for included in included_exits {
+            for (i, for_inclusion) in self.exits_for_inclusion.read().iter().enumerate() {
+                if included == for_inclusion {
+                    indices_to_delete.push(i);
+                }
+            }
+        }
+
+        let exits_for_inclusion = &mut self.exits_for_inclusion.write();
+        for i in indices_to_delete {
+            exits_for_inclusion.remove(i);
+        }
+    }
+
     /// Accept some proposer slashing and queue it for inclusion in an appropriate block.
     pub fn receive_proposer_slashing_for_inclusion(&self, proposer_slashing: ProposerSlashing) {
-        // TODO: proposer_slashings are not check for validity; check them.
+        // TODO: proposer_slashings are not checked for validity; check them.
+        //
+        // https://github.com/sigp/lighthouse/issues/276
         self.proposer_slashings_for_inclusion
             .write()
             .push(proposer_slashing);
@@ -413,6 +461,8 @@ where
     /// Return a vec of proposer slashings suitable for inclusion in some block.
     pub fn get_proposer_slashings_for_block(&self) -> Vec<ProposerSlashing> {
         // TODO: proposer_slashings are indiscriminately included; check them for validity.
+        //
+        // https://github.com/sigp/lighthouse/issues/275
         self.proposer_slashings_for_inclusion.read().clone()
     }
 
@@ -425,6 +475,8 @@ where
         included_proposer_slashings: &[ProposerSlashing],
     ) {
         // TODO: method does not take forks into account; consider this.
+        //
+        // https://github.com/sigp/lighthouse/issues/275
         let mut indices_to_delete = vec![];
 
         for included in included_proposer_slashings {
@@ -448,7 +500,9 @@ where
 
     /// Accept some attester slashing and queue it for inclusion in an appropriate block.
     pub fn receive_attester_slashing_for_inclusion(&self, attester_slashing: AttesterSlashing) {
-        // TODO: attester_slashings are not check for validity; check them.
+        // TODO: attester_slashings are not checked for validity; check them.
+        //
+        // https://github.com/sigp/lighthouse/issues/276
         self.attester_slashings_for_inclusion
             .write()
             .push(attester_slashing);
@@ -457,6 +511,8 @@ where
     /// Return a vec of attester slashings suitable for inclusion in some block.
     pub fn get_attester_slashings_for_block(&self) -> Vec<AttesterSlashing> {
         // TODO: attester_slashings are indiscriminately included; check them for validity.
+        //
+        // https://github.com/sigp/lighthouse/issues/275
         self.attester_slashings_for_inclusion.read().clone()
     }
 
@@ -469,6 +525,8 @@ where
         included_attester_slashings: &[AttesterSlashing],
     ) {
         // TODO: method does not take forks into account; consider this.
+        //
+        // https://github.com/sigp/lighthouse/issues/275
         let mut indices_to_delete = vec![];
 
         for included in included_attester_slashings {
@@ -678,7 +736,7 @@ where
                 attester_slashings: self.get_attester_slashings_for_block(),
                 attestations,
                 deposits: self.get_deposits_for_block(),
-                exits: vec![],
+                exits: self.get_exits_for_block(),
             },
         };
 
diff --git a/beacon_node/beacon_chain/test_harness/specs/validator_registry.yaml b/beacon_node/beacon_chain/test_harness/specs/validator_registry.yaml
index 56dd66558..a6021dbe5 100644
--- a/beacon_node/beacon_chain/test_harness/specs/validator_registry.yaml
+++ b/beacon_node/beacon_chain/test_harness/specs/validator_registry.yaml
@@ -18,6 +18,10 @@ test_cases:
           amount: 32
         - slot: 5
           amount: 32
+      exits:
+          # At slot 10, submit an exit for validator #50.
+        - slot: 10
+          validator_index: 50
       proposer_slashings:
           # At slot 2, trigger a proposer slashing for validator #42.
         - slot: 2
@@ -39,4 +43,5 @@ test_cases:
           num_validators: 1003
           slashed_validators: [11, 12, 13, 14, 42]
           exited_validators: []
+          exit_initiated_validators: [50]
 
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 ed5fddabf..105a6d6e1 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
@@ -280,6 +280,15 @@ impl BeaconChainHarness {
         }
     }
 
+    /// Submit an exit to the `BeaconChain` for inclusion in some block.
+    ///
+    /// Note: the `ValidatorHarness` for this validator continues to exist. Once it is exited it
+    /// will stop receiving duties from the beacon chain and just do nothing when prompted to
+    /// produce/attest.
+    pub fn add_exit(&mut self, exit: Exit) {
+        self.beacon_chain.receive_exit_for_inclusion(exit);
+    }
+
     /// Submit a proposer slashing to the `BeaconChain` for inclusion in some block.
     pub fn add_proposer_slashing(&mut self, proposer_slashing: ProposerSlashing) {
         self.beacon_chain
diff --git a/beacon_node/beacon_chain/test_harness/src/test_case/mod.rs b/beacon_node/beacon_chain/test_harness/src/test_case.rs
similarity index 86%
rename from beacon_node/beacon_chain/test_harness/src/test_case/mod.rs
rename to beacon_node/beacon_chain/test_harness/src/test_case.rs
index ae438a5d5..1a5ebfbe1 100644
--- a/beacon_node/beacon_chain/test_harness/src/test_case/mod.rs
+++ b/beacon_node/beacon_chain/test_harness/src/test_case.rs
@@ -4,6 +4,7 @@
 use crate::beacon_chain_harness::BeaconChainHarness;
 use beacon_chain::CheckPoint;
 use log::{info, warn};
+use ssz::TreeHash;
 use types::*;
 use types::{
     attester_slashing::AttesterSlashingBuilder, proposer_slashing::ProposerSlashingBuilder,
@@ -121,6 +122,20 @@ impl TestCase {
                 }
             }
 
+            // Feed exits to the BeaconChain.
+            if let Some(ref exits) = self.config.exits {
+                for (slot, validator_index) in exits {
+                    if *slot == slot_height {
+                        info!(
+                            "Including exit at slot height {} for validator {}.",
+                            slot_height, validator_index
+                        );
+                        let exit = build_exit(&harness, *validator_index);
+                        harness.add_exit(exit);
+                    }
+                }
+            }
+
             // Build a block or skip a slot.
             match self.config.skip_slots {
                 Some(ref skip_slots) if skip_slots.contains(&slot_height) => {
@@ -185,6 +200,33 @@ impl TestCase {
     }
 }
 
+fn build_exit(harness: &BeaconChainHarness, validator_index: u64) -> Exit {
+    let epoch = harness
+        .beacon_chain
+        .state
+        .read()
+        .current_epoch(&harness.spec);
+
+    let mut exit = Exit {
+        epoch,
+        validator_index,
+        signature: Signature::empty_signature(),
+    };
+
+    let message = exit.hash_tree_root();
+
+    exit.signature = harness
+        .validator_sign(
+            validator_index as usize,
+            &message[..],
+            epoch,
+            harness.spec.domain_exit,
+        )
+        .expect("Unable to sign Exit");
+
+    exit
+}
+
 /// Builds an `AttesterSlashing` for some `validator_indices`.
 ///
 /// Signs the message using a `BeaconChainHarness`.
diff --git a/beacon_node/beacon_chain/test_harness/src/test_case/config.rs b/beacon_node/beacon_chain/test_harness/src/test_case/config.rs
index 440c76cd2..4034c8ddc 100644
--- a/beacon_node/beacon_chain/test_harness/src/test_case/config.rs
+++ b/beacon_node/beacon_chain/test_harness/src/test_case/config.rs
@@ -3,9 +3,13 @@ use bls::create_proof_of_possession;
 use types::*;
 use yaml_rust::Yaml;
 
-pub type DepositTuple = (u64, Deposit, Keypair);
-pub type ProposerSlashingTuple = (u64, u64);
-pub type AttesterSlashingTuple = (u64, Vec<u64>);
+pub type ValidatorIndex = u64;
+pub type ValidatorIndices = Vec<u64>;
+
+pub type DepositTuple = (SlotHeight, Deposit, Keypair);
+pub type ExitTuple = (SlotHeight, ValidatorIndex);
+pub type ProposerSlashingTuple = (SlotHeight, ValidatorIndex);
+pub type AttesterSlashingTuple = (SlotHeight, ValidatorIndices);
 
 /// Defines the execution of a `BeaconStateHarness` across a series of slots.
 #[derive(Debug)]
@@ -24,6 +28,8 @@ pub struct Config {
     pub proposer_slashings: Option<Vec<ProposerSlashingTuple>>,
     /// Attester slashings to be including during execution.
     pub attester_slashings: Option<Vec<AttesterSlashingTuple>>,
+    /// Exits to be including during execution.
+    pub exits: Option<Vec<ExitTuple>>,
 }
 
 impl Config {
@@ -40,10 +46,26 @@ impl Config {
             deposits: parse_deposits(&yaml),
             proposer_slashings: parse_proposer_slashings(&yaml),
             attester_slashings: parse_attester_slashings(&yaml),
+            exits: parse_exits(&yaml),
         }
     }
 }
 
+/// Parse the `attester_slashings` section of the YAML document.
+fn parse_exits(yaml: &Yaml) -> Option<Vec<ExitTuple>> {
+    let mut tuples = vec![];
+
+    for exit in yaml["exits"].as_vec()? {
+        let slot = as_u64(exit, "slot").expect("Incomplete exit (slot)");
+        let validator_index =
+            as_u64(exit, "validator_index").expect("Incomplete exit (validator_index)");
+
+        tuples.push((SlotHeight::from(slot), validator_index));
+    }
+
+    Some(tuples)
+}
+
 /// Parse the `attester_slashings` section of the YAML document.
 fn parse_attester_slashings(yaml: &Yaml) -> Option<Vec<AttesterSlashingTuple>> {
     let mut slashings = vec![];
@@ -53,7 +75,7 @@ fn parse_attester_slashings(yaml: &Yaml) -> Option<Vec<AttesterSlashingTuple>> {
         let validator_indices = as_vec_u64(slashing, "validator_indices")
             .expect("Incomplete attester_slashing (validator_indices)");
 
-        slashings.push((slot, validator_indices));
+        slashings.push((SlotHeight::from(slot), validator_indices));
     }
 
     Some(slashings)
@@ -68,7 +90,7 @@ fn parse_proposer_slashings(yaml: &Yaml) -> Option<Vec<ProposerSlashingTuple>> {
         let validator_index = as_u64(slashing, "validator_index")
             .expect("Incomplete proposer slashing (validator_index)");
 
-        slashings.push((slot, validator_index));
+        slashings.push((SlotHeight::from(slot), validator_index));
     }
 
     Some(slashings)
@@ -102,7 +124,7 @@ fn parse_deposits(yaml: &Yaml) -> Option<Vec<DepositTuple>> {
             },
         };
 
-        deposits.push((slot, deposit, keypair));
+        deposits.push((SlotHeight::from(slot), deposit, keypair));
     }
 
     Some(deposits)
diff --git a/beacon_node/beacon_chain/test_harness/src/test_case/state_check.rs b/beacon_node/beacon_chain/test_harness/src/test_case/state_check.rs
index 3126dcfea..b52dc7d6b 100644
--- a/beacon_node/beacon_chain/test_harness/src/test_case/state_check.rs
+++ b/beacon_node/beacon_chain/test_harness/src/test_case/state_check.rs
@@ -13,8 +13,10 @@ pub struct StateCheck {
     pub num_validators: Option<usize>,
     /// A list of validator indices which have been penalized. Must be in ascending order.
     pub slashed_validators: Option<Vec<u64>>,
-    /// A list of validator indices which have been exited. Must be in ascending order.
+    /// A list of validator indices which have been fully exited. Must be in ascending order.
     pub exited_validators: Option<Vec<u64>>,
+    /// A list of validator indices which have had an exit initiated. Must be in ascending order.
+    pub exit_initiated_validators: Option<Vec<u64>>,
 }
 
 impl StateCheck {
@@ -27,6 +29,7 @@ impl StateCheck {
             num_validators: as_usize(&yaml, "num_validators"),
             slashed_validators: as_vec_u64(&yaml, "slashed_validators"),
             exited_validators: as_vec_u64(&yaml, "exited_validators"),
+            exit_initiated_validators: as_vec_u64(&yaml, "exit_initiated_validators"),
         }
     }
 
@@ -40,6 +43,7 @@ impl StateCheck {
 
         info!("Running state check for slot height {}.", self.slot);
 
+        // Check the state slot.
         assert_eq!(
             self.slot,
             state.slot - spec.genesis_epoch.start_slot(spec.slots_per_epoch),
@@ -55,6 +59,7 @@ impl StateCheck {
             info!("OK: num_validators = {}.", num_validators);
         }
 
+        // Check for slashed validators.
         if let Some(ref slashed_validators) = self.slashed_validators {
             let actually_slashed_validators: Vec<u64> = state
                 .validator_registry
@@ -75,6 +80,7 @@ impl StateCheck {
             info!("OK: slashed_validators = {:?}.", slashed_validators);
         }
 
+        // Check for exited validators.
         if let Some(ref exited_validators) = self.exited_validators {
             let actually_exited_validators: Vec<u64> = state
                 .validator_registry
@@ -94,5 +100,29 @@ impl StateCheck {
             );
             info!("OK: exited_validators = {:?}.", exited_validators);
         }
+
+        // Check for validators that have initiated exit.
+        if let Some(ref exit_initiated_validators) = self.exit_initiated_validators {
+            let actual: Vec<u64> = state
+                .validator_registry
+                .iter()
+                .enumerate()
+                .filter_map(|(i, validator)| {
+                    if validator.has_initiated_exit() {
+                        Some(i as u64)
+                    } else {
+                        None
+                    }
+                })
+                .collect();
+            assert_eq!(
+                actual, *exit_initiated_validators,
+                "Exit initiated validators != expected."
+            );
+            info!(
+                "OK: exit_initiated_validators = {:?}.",
+                exit_initiated_validators
+            );
+        }
     }
 }
diff --git a/beacon_node/db/src/stores/beacon_block_store.rs b/beacon_node/db/src/stores/beacon_block_store.rs
index 8a1fc2b0d..bd5149cfd 100644
--- a/beacon_node/db/src/stores/beacon_block_store.rs
+++ b/beacon_node/db/src/stores/beacon_block_store.rs
@@ -134,9 +134,9 @@ mod tests {
         let store = BeaconBlockStore::new(db.clone());
 
         let ssz = "definitly not a valid block".as_bytes();
-        let hash = &Hash256::from("some hash".as_bytes());
+        let hash = &Hash256::from([0xAA; 32]);
 
-        db.put(DB_COLUMN, hash, ssz).unwrap();
+        db.put(DB_COLUMN, hash.as_bytes(), ssz).unwrap();
         assert_eq!(
             store.block_at_slot(hash, Slot::from(42_u64)),
             Err(BeaconBlockAtSlotError::DBError(
@@ -151,10 +151,10 @@ mod tests {
         let store = BeaconBlockStore::new(db.clone());
 
         let ssz = "some bytes".as_bytes();
-        let hash = &Hash256::from("some hash".as_bytes());
-        let other_hash = &Hash256::from("another hash".as_bytes());
+        let hash = &Hash256::from([0xAA; 32]);
+        let other_hash = &Hash256::from([0xBB; 32]);
 
-        db.put(DB_COLUMN, hash, ssz).unwrap();
+        db.put(DB_COLUMN, hash.as_bytes(), ssz).unwrap();
         assert_eq!(
             store.block_at_slot(other_hash, Slot::from(42_u64)),
             Err(BeaconBlockAtSlotError::UnknownBeaconBlock(*other_hash))
@@ -169,18 +169,15 @@ mod tests {
         let thread_count = 10;
         let write_count = 10;
 
-        // We're expecting the product of these numbers to fit in one byte.
-        assert!(thread_count * write_count <= 255);
-
         let mut handles = vec![];
         for t in 0..thread_count {
             let wc = write_count;
             let bs = bs.clone();
             let handle = thread::spawn(move || {
                 for w in 0..wc {
-                    let key = (t * w) as u8;
+                    let key = t * w;
                     let val = 42;
-                    bs.put(&[key][..].into(), &vec![val]).unwrap();
+                    bs.put(&Hash256::from_low_u64_le(key), &vec![val]).unwrap();
                 }
             });
             handles.push(handle);
@@ -192,9 +189,9 @@ mod tests {
 
         for t in 0..thread_count {
             for w in 0..write_count {
-                let key = (t * w) as u8;
-                assert!(bs.exists(&[key][..].into()).unwrap());
-                let val = bs.get(&[key][..].into()).unwrap().unwrap();
+                let key = t * w;
+                assert!(bs.exists(&Hash256::from_low_u64_le(key)).unwrap());
+                let val = bs.get(&Hash256::from_low_u64_le(key)).unwrap().unwrap();
                 assert_eq!(vec![42], val);
             }
         }
@@ -208,19 +205,20 @@ mod tests {
 
         // Specify test block parameters.
         let hashes = [
-            Hash256::from(&[0; 32][..]),
-            Hash256::from(&[1; 32][..]),
-            Hash256::from(&[2; 32][..]),
-            Hash256::from(&[3; 32][..]),
-            Hash256::from(&[4; 32][..]),
+            Hash256::from([0; 32]),
+            Hash256::from([1; 32]),
+            Hash256::from([2; 32]),
+            Hash256::from([3; 32]),
+            Hash256::from([4; 32]),
         ];
         let parent_hashes = [
-            Hash256::from(&[255; 32][..]), // Genesis block.
-            Hash256::from(&[0; 32][..]),
-            Hash256::from(&[1; 32][..]),
-            Hash256::from(&[2; 32][..]),
-            Hash256::from(&[3; 32][..]),
+            Hash256::from([255; 32]), // Genesis block.
+            Hash256::from([0; 32]),
+            Hash256::from([1; 32]),
+            Hash256::from([2; 32]),
+            Hash256::from([3; 32]),
         ];
+        let unknown_hash = Hash256::from([101; 32]); // different from all above
         let slots: Vec<Slot> = vec![0, 1, 3, 4, 5].iter().map(|x| Slot::new(*x)).collect();
 
         // Generate a vec of random blocks and store them in the DB.
@@ -233,7 +231,7 @@ mod tests {
             block.slot = slots[i];
 
             let ssz = ssz_encode(&block);
-            db.put(DB_COLUMN, &hashes[i], &ssz).unwrap();
+            db.put(DB_COLUMN, hashes[i].as_bytes(), &ssz).unwrap();
 
             blocks.push(block);
         }
@@ -255,11 +253,10 @@ mod tests {
         let ssz = bs.block_at_slot(&hashes[4], Slot::new(6)).unwrap();
         assert_eq!(ssz, None);
 
-        let bad_hash = &Hash256::from("unknown".as_bytes());
-        let ssz = bs.block_at_slot(bad_hash, Slot::new(2));
+        let ssz = bs.block_at_slot(&unknown_hash, Slot::new(2));
         assert_eq!(
             ssz,
-            Err(BeaconBlockAtSlotError::UnknownBeaconBlock(*bad_hash))
+            Err(BeaconBlockAtSlotError::UnknownBeaconBlock(unknown_hash))
         );
     }
 }
diff --git a/beacon_node/db/src/stores/macros.rs b/beacon_node/db/src/stores/macros.rs
index 36b8aef8e..6c53e40ee 100644
--- a/beacon_node/db/src/stores/macros.rs
+++ b/beacon_node/db/src/stores/macros.rs
@@ -2,25 +2,25 @@ macro_rules! impl_crud_for_store {
     ($store: ident, $db_column: expr) => {
         impl<T: ClientDB> $store<T> {
             pub fn put(&self, hash: &Hash256, ssz: &[u8]) -> Result<(), DBError> {
-                self.db.put($db_column, hash, ssz)
+                self.db.put($db_column, hash.as_bytes(), ssz)
             }
 
             pub fn get(&self, hash: &Hash256) -> Result<Option<Vec<u8>>, DBError> {
-                self.db.get($db_column, hash)
+                self.db.get($db_column, hash.as_bytes())
             }
 
             pub fn exists(&self, hash: &Hash256) -> Result<bool, DBError> {
-                self.db.exists($db_column, hash)
+                self.db.exists($db_column, hash.as_bytes())
             }
 
             pub fn delete(&self, hash: &Hash256) -> Result<(), DBError> {
-                self.db.delete($db_column, hash)
+                self.db.delete($db_column, hash.as_bytes())
             }
         }
     };
 }
 
-#[allow(unused_macros)]
+#[cfg(test)]
 macro_rules! test_crud_for_store {
     ($store: ident, $db_column: expr) => {
         #[test]
@@ -29,10 +29,10 @@ macro_rules! test_crud_for_store {
             let store = $store::new(db.clone());
 
             let ssz = "some bytes".as_bytes();
-            let hash = &Hash256::from("some hash".as_bytes());
+            let hash = &Hash256::from([0xAA; 32]);
 
             store.put(hash, ssz).unwrap();
-            assert_eq!(db.get(DB_COLUMN, hash).unwrap().unwrap(), ssz);
+            assert_eq!(db.get(DB_COLUMN, hash.as_bytes()).unwrap().unwrap(), ssz);
         }
 
         #[test]
@@ -41,9 +41,9 @@ macro_rules! test_crud_for_store {
             let store = $store::new(db.clone());
 
             let ssz = "some bytes".as_bytes();
-            let hash = &Hash256::from("some hash".as_bytes());
+            let hash = &Hash256::from([0xAA; 32]);
 
-            db.put(DB_COLUMN, hash, ssz).unwrap();
+            db.put(DB_COLUMN, hash.as_bytes(), ssz).unwrap();
             assert_eq!(store.get(hash).unwrap().unwrap(), ssz);
         }
 
@@ -53,10 +53,10 @@ macro_rules! test_crud_for_store {
             let store = $store::new(db.clone());
 
             let ssz = "some bytes".as_bytes();
-            let hash = &Hash256::from("some hash".as_bytes());
-            let other_hash = &Hash256::from("another hash".as_bytes());
+            let hash = &Hash256::from([0xAA; 32]);
+            let other_hash = &Hash256::from([0xBB; 32]);
 
-            db.put(DB_COLUMN, other_hash, ssz).unwrap();
+            db.put(DB_COLUMN, other_hash.as_bytes(), ssz).unwrap();
             assert_eq!(store.get(hash).unwrap(), None);
         }
 
@@ -66,9 +66,9 @@ macro_rules! test_crud_for_store {
             let store = $store::new(db.clone());
 
             let ssz = "some bytes".as_bytes();
-            let hash = &Hash256::from("some hash".as_bytes());
+            let hash = &Hash256::from([0xAA; 32]);
 
-            db.put(DB_COLUMN, hash, ssz).unwrap();
+            db.put(DB_COLUMN, hash.as_bytes(), ssz).unwrap();
             assert!(store.exists(hash).unwrap());
         }
 
@@ -78,10 +78,10 @@ macro_rules! test_crud_for_store {
             let store = $store::new(db.clone());
 
             let ssz = "some bytes".as_bytes();
-            let hash = &Hash256::from("some hash".as_bytes());
-            let other_hash = &Hash256::from("another hash".as_bytes());
+            let hash = &Hash256::from([0xAA; 32]);
+            let other_hash = &Hash256::from([0xBB; 32]);
 
-            db.put(DB_COLUMN, hash, ssz).unwrap();
+            db.put(DB_COLUMN, hash.as_bytes(), ssz).unwrap();
             assert!(!store.exists(other_hash).unwrap());
         }
 
@@ -91,13 +91,13 @@ macro_rules! test_crud_for_store {
             let store = $store::new(db.clone());
 
             let ssz = "some bytes".as_bytes();
-            let hash = &Hash256::from("some hash".as_bytes());
+            let hash = &Hash256::from([0xAA; 32]);
 
-            db.put(DB_COLUMN, hash, ssz).unwrap();
-            assert!(db.exists(DB_COLUMN, hash).unwrap());
+            db.put(DB_COLUMN, hash.as_bytes(), ssz).unwrap();
+            assert!(db.exists(DB_COLUMN, hash.as_bytes()).unwrap());
 
             store.delete(hash).unwrap();
-            assert!(!db.exists(DB_COLUMN, hash).unwrap());
+            assert!(!db.exists(DB_COLUMN, hash.as_bytes()).unwrap());
         }
     };
 }
diff --git a/beacon_node/db/src/stores/pow_chain_store.rs b/beacon_node/db/src/stores/pow_chain_store.rs
index a7c77bab5..5c8b97907 100644
--- a/beacon_node/db/src/stores/pow_chain_store.rs
+++ b/beacon_node/db/src/stores/pow_chain_store.rs
@@ -37,7 +37,7 @@ mod tests {
         let db = Arc::new(MemoryDB::open());
         let store = PoWChainStore::new(db.clone());
 
-        let hash = &Hash256::from("some hash".as_bytes()).to_vec();
+        let hash = &Hash256::from([0xAA; 32]).as_bytes().to_vec();
         store.put_block_hash(hash).unwrap();
 
         assert!(db.exists(DB_COLUMN, hash).unwrap());
@@ -48,7 +48,7 @@ mod tests {
         let db = Arc::new(MemoryDB::open());
         let store = PoWChainStore::new(db.clone());
 
-        let hash = &Hash256::from("some hash".as_bytes()).to_vec();
+        let hash = &Hash256::from([0xAA; 32]).as_bytes().to_vec();
         db.put(DB_COLUMN, hash, &[0]).unwrap();
 
         assert!(store.block_hash_exists(hash).unwrap());
@@ -59,8 +59,8 @@ mod tests {
         let db = Arc::new(MemoryDB::open());
         let store = PoWChainStore::new(db.clone());
 
-        let hash = &Hash256::from("some hash".as_bytes()).to_vec();
-        let other_hash = &Hash256::from("another hash".as_bytes()).to_vec();
+        let hash = &Hash256::from([0xAA; 32]).as_bytes().to_vec();
+        let other_hash = &Hash256::from([0xBB; 32]).as_bytes().to_vec();
         db.put(DB_COLUMN, hash, &[0]).unwrap();
 
         assert!(!store.block_hash_exists(other_hash).unwrap());
diff --git a/docs/lighthouse.md b/docs/lighthouse.md
index 8ca2387f8..16da13b56 100644
--- a/docs/lighthouse.md
+++ b/docs/lighthouse.md
@@ -67,7 +67,7 @@ into individual crates wherever possible.
 
 Generally, tests can be kept in the same file, as is typical in Rust.
 Integration tests should be placed in the `tests` directory in the crate's
-root.  Particularity large (line-count) tests should be placed into a separate
+root.  Particularly large (line-count) tests should be placed into a separate
 file.
 
 A function is not considered complete until a test exists for it. We produce
diff --git a/docs/onboarding.md b/docs/onboarding.md
index 8af3b0a83..275f95484 100644
--- a/docs/onboarding.md
+++ b/docs/onboarding.md
@@ -122,7 +122,7 @@ project.
 * **Module**: A collection of items: functions, structs, traits, and even other
 modules. Modules allow you to hierarchically split code into logical units
 and manage visibility.
-* **Attribute**: Metadaata applied to some module, crate or item.
+* **Attribute**: Metadata applied to some module, crate or item.
 * **Macros**: Macros are powerful meta-programming statements that get expanded
 into source code that gets compiled with the rest of the code (Unlike `C`
 macros that are pre-processed, Rust macros form an Abstract Syntax Tree).
@@ -185,7 +185,7 @@ check your code.
 | Function / Method   | ``snake_case``                 |
 | Macro Names         | ``snake_case``                 |
 | Constants           | ``SCREAMING_SNAKE_CASE``       |
-| Forbidden name      | Trialing Underscore: ``name_`` |
+| Forbidden name      | Trailing Underscore: ``name_`` |
 
 Other general rust docs:
 
diff --git a/eth2/fork_choice/src/bitwise_lmd_ghost.rs b/eth2/fork_choice/src/bitwise_lmd_ghost.rs
index c13aacbf4..d50471f56 100644
--- a/eth2/fork_choice/src/bitwise_lmd_ghost.rs
+++ b/eth2/fork_choice/src/bitwise_lmd_ghost.rs
@@ -210,7 +210,7 @@ where
 
             trace!("Child vote length: {}", votes.len());
             for (candidate, votes) in votes.iter() {
-                let candidate_bit: BitVec = BitVec::from_bytes(&candidate);
+                let candidate_bit: BitVec = BitVec::from_bytes(candidate.as_bytes());
 
                 // if the bitmasks don't match, exclude candidate
                 if !bitmask.iter().eq(candidate_bit.iter().take(bit)) {
diff --git a/eth2/state_processing/src/block_processable.rs b/eth2/state_processing/src/block_processable.rs
new file mode 100644
index 000000000..a54bb96d4
--- /dev/null
+++ b/eth2/state_processing/src/block_processable.rs
@@ -0,0 +1,455 @@
+use self::verify_slashable_attestation::verify_slashable_attestation;
+use crate::SlotProcessingError;
+use hashing::hash;
+use int_to_bytes::int_to_bytes32;
+use log::{debug, trace};
+use ssz::{ssz_encode, TreeHash};
+use types::*;
+
+mod verify_slashable_attestation;
+
+const PHASE_0_CUSTODY_BIT: bool = false;
+
+#[derive(Debug, PartialEq)]
+pub enum Error {
+    DBError(String),
+    StateAlreadyTransitioned,
+    PresentSlotIsNone,
+    UnableToDecodeBlock,
+    MissingParentState(Hash256),
+    InvalidParentState(Hash256),
+    MissingBeaconBlock(Hash256),
+    InvalidBeaconBlock(Hash256),
+    MissingParentBlock(Hash256),
+    StateSlotMismatch,
+    BadBlockSignature,
+    BadRandaoSignature,
+    MaxProposerSlashingsExceeded,
+    BadProposerSlashing,
+    MaxAttesterSlashingsExceed,
+    MaxAttestationsExceeded,
+    BadAttesterSlashing,
+    InvalidAttestation(AttestationValidationError),
+    NoBlockRoot,
+    MaxDepositsExceeded,
+    BadDeposit,
+    MaxExitsExceeded,
+    BadExit,
+    BadCustodyReseeds,
+    BadCustodyChallenges,
+    BadCustodyResponses,
+    BeaconStateError(BeaconStateError),
+    SlotProcessingError(SlotProcessingError),
+}
+
+#[derive(Debug, PartialEq)]
+pub enum AttestationValidationError {
+    IncludedTooEarly,
+    IncludedTooLate,
+    WrongJustifiedSlot,
+    WrongJustifiedRoot,
+    BadLatestCrosslinkRoot,
+    BadSignature,
+    ShardBlockRootNotZero,
+    NoBlockRoot,
+    BeaconStateError(BeaconStateError),
+}
+
+macro_rules! ensure {
+    ($condition: expr, $result: expr) => {
+        if !$condition {
+            return Err($result);
+        }
+    };
+}
+
+pub trait BlockProcessable {
+    fn per_block_processing(&mut self, block: &BeaconBlock, spec: &ChainSpec) -> Result<(), Error>;
+    fn per_block_processing_without_verifying_block_signature(
+        &mut self,
+        block: &BeaconBlock,
+        spec: &ChainSpec,
+    ) -> Result<(), Error>;
+}
+
+impl BlockProcessable for BeaconState {
+    fn per_block_processing(&mut self, block: &BeaconBlock, spec: &ChainSpec) -> Result<(), Error> {
+        per_block_processing_signature_optional(self, block, true, spec)
+    }
+
+    fn per_block_processing_without_verifying_block_signature(
+        &mut self,
+        block: &BeaconBlock,
+        spec: &ChainSpec,
+    ) -> Result<(), Error> {
+        per_block_processing_signature_optional(self, block, false, spec)
+    }
+}
+
+fn per_block_processing_signature_optional(
+    mut state: &mut BeaconState,
+    block: &BeaconBlock,
+    verify_block_signature: bool,
+    spec: &ChainSpec,
+) -> Result<(), Error> {
+    ensure!(block.slot == state.slot, Error::StateSlotMismatch);
+
+    // Building the previous epoch could be delayed until an attestation from a previous epoch is
+    // included. This is left for future optimisation.
+    state.build_epoch_cache(RelativeEpoch::Previous, spec)?;
+    state.build_epoch_cache(RelativeEpoch::Current, spec)?;
+
+    /*
+     * Proposer Signature
+     */
+    let block_proposer_index = state.get_beacon_proposer_index(block.slot, spec)?;
+    let block_proposer = &state.validator_registry[block_proposer_index];
+
+    if verify_block_signature {
+        ensure!(
+            bls_verify(
+                &block_proposer.pubkey,
+                &block.proposal_root(spec)[..],
+                &block.signature,
+                get_domain(&state.fork, state.current_epoch(spec), spec.domain_proposal)
+            ),
+            Error::BadBlockSignature
+        );
+    }
+
+    /*
+     * RANDAO
+     */
+    ensure!(
+        bls_verify(
+            &block_proposer.pubkey,
+            &int_to_bytes32(state.current_epoch(spec).as_u64()),
+            &block.randao_reveal,
+            get_domain(&state.fork, state.current_epoch(spec), spec.domain_randao)
+        ),
+        Error::BadRandaoSignature
+    );
+
+    // TODO: check this is correct.
+    let new_mix = {
+        let mut mix = state.latest_randao_mixes
+            [state.slot.as_usize() % spec.latest_randao_mixes_length]
+            .as_bytes()
+            .to_vec();
+        mix.append(&mut ssz_encode(&block.randao_reveal));
+        Hash256::from_slice(&hash(&mix)[..])
+    };
+
+    state.latest_randao_mixes[state.slot.as_usize() % spec.latest_randao_mixes_length] = new_mix;
+
+    /*
+     * Eth1 data
+     */
+    // TODO: Eth1 data processing.
+
+    /*
+     * Proposer slashings
+     */
+    ensure!(
+        block.body.proposer_slashings.len() as u64 <= spec.max_proposer_slashings,
+        Error::MaxProposerSlashingsExceeded
+    );
+    for proposer_slashing in &block.body.proposer_slashings {
+        let proposer = state
+            .validator_registry
+            .get(proposer_slashing.proposer_index as usize)
+            .ok_or(Error::BadProposerSlashing)?;
+        ensure!(
+            proposer_slashing.proposal_data_1.slot == proposer_slashing.proposal_data_2.slot,
+            Error::BadProposerSlashing
+        );
+        ensure!(
+            proposer_slashing.proposal_data_1.shard == proposer_slashing.proposal_data_2.shard,
+            Error::BadProposerSlashing
+        );
+        ensure!(
+            proposer_slashing.proposal_data_1.block_root
+                != proposer_slashing.proposal_data_2.block_root,
+            Error::BadProposerSlashing
+        );
+        ensure!(
+            proposer.penalized_epoch > state.current_epoch(spec),
+            Error::BadProposerSlashing
+        );
+        ensure!(
+            bls_verify(
+                &proposer.pubkey,
+                &proposer_slashing.proposal_data_1.hash_tree_root(),
+                &proposer_slashing.proposal_signature_1,
+                get_domain(
+                    &state.fork,
+                    proposer_slashing
+                        .proposal_data_1
+                        .slot
+                        .epoch(spec.epoch_length),
+                    spec.domain_proposal
+                )
+            ),
+            Error::BadProposerSlashing
+        );
+        ensure!(
+            bls_verify(
+                &proposer.pubkey,
+                &proposer_slashing.proposal_data_2.hash_tree_root(),
+                &proposer_slashing.proposal_signature_2,
+                get_domain(
+                    &state.fork,
+                    proposer_slashing
+                        .proposal_data_2
+                        .slot
+                        .epoch(spec.epoch_length),
+                    spec.domain_proposal
+                )
+            ),
+            Error::BadProposerSlashing
+        );
+        state.penalize_validator(proposer_slashing.proposer_index as usize, spec)?;
+    }
+
+    /*
+     * Attester slashings
+     */
+    ensure!(
+        block.body.attester_slashings.len() as u64 <= spec.max_attester_slashings,
+        Error::MaxAttesterSlashingsExceed
+    );
+    for attester_slashing in &block.body.attester_slashings {
+        verify_slashable_attestation(&mut state, &attester_slashing, spec)?;
+    }
+
+    /*
+     * Attestations
+     */
+    ensure!(
+        block.body.attestations.len() as u64 <= spec.max_attestations,
+        Error::MaxAttestationsExceeded
+    );
+
+    debug!("Verifying {} attestations.", block.body.attestations.len());
+
+    for attestation in &block.body.attestations {
+        validate_attestation(&state, attestation, spec)?;
+
+        let pending_attestation = PendingAttestation {
+            data: attestation.data.clone(),
+            aggregation_bitfield: attestation.aggregation_bitfield.clone(),
+            custody_bitfield: attestation.custody_bitfield.clone(),
+            inclusion_slot: state.slot,
+        };
+        state.latest_attestations.push(pending_attestation);
+    }
+
+    /*
+     * Deposits
+     */
+    ensure!(
+        block.body.deposits.len() as u64 <= spec.max_deposits,
+        Error::MaxDepositsExceeded
+    );
+
+    // TODO: verify deposit merkle branches.
+    for deposit in &block.body.deposits {
+        debug!(
+            "Processing deposit for pubkey {:?}",
+            deposit.deposit_data.deposit_input.pubkey
+        );
+        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::BadDeposit)?;
+    }
+
+    /*
+     * Exits
+     */
+    ensure!(
+        block.body.exits.len() as u64 <= spec.max_exits,
+        Error::MaxExitsExceeded
+    );
+
+    for exit in &block.body.exits {
+        let validator = state
+            .validator_registry
+            .get(exit.validator_index as usize)
+            .ok_or(Error::BadExit)?;
+        ensure!(
+            validator.exit_epoch
+                > state.get_entry_exit_effect_epoch(state.current_epoch(spec), spec),
+            Error::BadExit
+        );
+        ensure!(state.current_epoch(spec) >= exit.epoch, Error::BadExit);
+        let exit_message = {
+            let exit_struct = Exit {
+                epoch: exit.epoch,
+                validator_index: exit.validator_index,
+                signature: spec.empty_signature.clone(),
+            };
+            exit_struct.hash_tree_root()
+        };
+        ensure!(
+            bls_verify(
+                &validator.pubkey,
+                &exit_message,
+                &exit.signature,
+                get_domain(&state.fork, exit.epoch, spec.domain_exit)
+            ),
+            Error::BadProposerSlashing
+        );
+        state.initiate_validator_exit(exit.validator_index as usize);
+    }
+
+    debug!("State transition complete.");
+
+    Ok(())
+}
+
+pub fn validate_attestation(
+    state: &BeaconState,
+    attestation: &Attestation,
+    spec: &ChainSpec,
+) -> Result<(), AttestationValidationError> {
+    validate_attestation_signature_optional(state, attestation, spec, true)
+}
+
+pub fn validate_attestation_without_signature(
+    state: &BeaconState,
+    attestation: &Attestation,
+    spec: &ChainSpec,
+) -> Result<(), AttestationValidationError> {
+    validate_attestation_signature_optional(state, attestation, spec, false)
+}
+
+fn validate_attestation_signature_optional(
+    state: &BeaconState,
+    attestation: &Attestation,
+    spec: &ChainSpec,
+    verify_signature: bool,
+) -> Result<(), AttestationValidationError> {
+    trace!(
+        "validate_attestation_signature_optional: attestation epoch: {}",
+        attestation.data.slot.epoch(spec.epoch_length)
+    );
+    ensure!(
+        attestation.data.slot + spec.min_attestation_inclusion_delay <= state.slot,
+        AttestationValidationError::IncludedTooEarly
+    );
+    ensure!(
+        attestation.data.slot + spec.epoch_length >= state.slot,
+        AttestationValidationError::IncludedTooLate
+    );
+    if attestation.data.slot >= state.current_epoch_start_slot(spec) {
+        ensure!(
+            attestation.data.justified_epoch == state.justified_epoch,
+            AttestationValidationError::WrongJustifiedSlot
+        );
+    } else {
+        ensure!(
+            attestation.data.justified_epoch == state.previous_justified_epoch,
+            AttestationValidationError::WrongJustifiedSlot
+        );
+    }
+    ensure!(
+        attestation.data.justified_block_root
+            == *state
+                .get_block_root(
+                    attestation
+                        .data
+                        .justified_epoch
+                        .start_slot(spec.epoch_length),
+                    &spec
+                )
+                .ok_or(AttestationValidationError::NoBlockRoot)?,
+        AttestationValidationError::WrongJustifiedRoot
+    );
+    let potential_crosslink = Crosslink {
+        shard_block_root: attestation.data.shard_block_root,
+        epoch: attestation.data.slot.epoch(spec.epoch_length),
+    };
+    ensure!(
+        (attestation.data.latest_crosslink
+            == state.latest_crosslinks[attestation.data.shard as usize])
+            | (attestation.data.latest_crosslink == potential_crosslink),
+        AttestationValidationError::BadLatestCrosslinkRoot
+    );
+    if verify_signature {
+        let participants = state.get_attestation_participants(
+            &attestation.data,
+            &attestation.aggregation_bitfield,
+            spec,
+        )?;
+        trace!(
+            "slot: {}, shard: {}, participants: {:?}",
+            attestation.data.slot,
+            attestation.data.shard,
+            participants
+        );
+        let mut group_public_key = AggregatePublicKey::new();
+        for participant in participants {
+            group_public_key.add(&state.validator_registry[participant as usize].pubkey)
+        }
+        ensure!(
+            attestation.verify_signature(
+                &group_public_key,
+                PHASE_0_CUSTODY_BIT,
+                get_domain(
+                    &state.fork,
+                    attestation.data.slot.epoch(spec.epoch_length),
+                    spec.domain_attestation,
+                )
+            ),
+            AttestationValidationError::BadSignature
+        );
+    }
+    ensure!(
+        attestation.data.shard_block_root == spec.zero_hash,
+        AttestationValidationError::ShardBlockRootNotZero
+    );
+    Ok(())
+}
+
+fn get_domain(fork: &Fork, epoch: Epoch, domain_type: u64) -> u64 {
+    fork.get_domain(epoch, domain_type)
+}
+
+fn bls_verify(pubkey: &PublicKey, message: &[u8], signature: &Signature, domain: u64) -> bool {
+    signature.verify(message, domain, pubkey)
+}
+
+impl From<AttestationValidationError> for Error {
+    fn from(e: AttestationValidationError) -> Error {
+        Error::InvalidAttestation(e)
+    }
+}
+
+impl From<BeaconStateError> for Error {
+    fn from(e: BeaconStateError) -> Error {
+        Error::BeaconStateError(e)
+    }
+}
+
+impl From<SlotProcessingError> for Error {
+    fn from(e: SlotProcessingError) -> Error {
+        Error::SlotProcessingError(e)
+    }
+}
+
+impl From<BeaconStateError> for AttestationValidationError {
+    fn from(e: BeaconStateError) -> AttestationValidationError {
+        AttestationValidationError::BeaconStateError(e)
+    }
+}
diff --git a/eth2/state_processing/src/epoch_processable.rs b/eth2/state_processing/src/epoch_processable.rs
new file mode 100644
index 000000000..ff6f18113
--- /dev/null
+++ b/eth2/state_processing/src/epoch_processable.rs
@@ -0,0 +1,723 @@
+use integer_sqrt::IntegerSquareRoot;
+use log::{debug, trace};
+use rayon::prelude::*;
+use ssz::TreeHash;
+use std::collections::{HashMap, HashSet};
+use std::iter::FromIterator;
+use types::{
+    validator_registry::get_active_validator_indices, BeaconState, BeaconStateError, ChainSpec,
+    Crosslink, Epoch, Hash256, InclusionError, PendingAttestation, RelativeEpoch,
+};
+
+mod tests;
+
+macro_rules! safe_add_assign {
+    ($a: expr, $b: expr) => {
+        $a = $a.saturating_add($b);
+    };
+}
+macro_rules! safe_sub_assign {
+    ($a: expr, $b: expr) => {
+        $a = $a.saturating_sub($b);
+    };
+}
+
+#[derive(Debug, PartialEq)]
+pub enum Error {
+    UnableToDetermineProducer,
+    NoBlockRoots,
+    BaseRewardQuotientIsZero,
+    NoRandaoSeed,
+    BeaconStateError(BeaconStateError),
+    InclusionError(InclusionError),
+    WinningRootError(WinningRootError),
+}
+
+#[derive(Debug, PartialEq)]
+pub enum WinningRootError {
+    NoWinningRoot,
+    BeaconStateError(BeaconStateError),
+}
+
+#[derive(Clone)]
+pub struct WinningRoot {
+    pub shard_block_root: Hash256,
+    pub attesting_validator_indices: Vec<usize>,
+    pub total_balance: u64,
+    pub total_attesting_balance: u64,
+}
+
+pub trait EpochProcessable {
+    fn per_epoch_processing(&mut self, spec: &ChainSpec) -> Result<(), Error>;
+}
+
+impl EpochProcessable for BeaconState {
+    // Cyclomatic complexity is ignored. It would be ideal to split this function apart, however it
+    // remains monolithic to allow for easier spec updates. Once the spec is more stable we can
+    // optimise.
+    #[allow(clippy::cyclomatic_complexity)]
+    fn per_epoch_processing(&mut self, spec: &ChainSpec) -> Result<(), Error> {
+        let current_epoch = self.current_epoch(spec);
+        let previous_epoch = self.previous_epoch(spec);
+        let next_epoch = self.next_epoch(spec);
+
+        debug!(
+            "Starting per-epoch processing on epoch {}...",
+            self.current_epoch(spec)
+        );
+
+        // Ensure all of the caches are built.
+        self.build_epoch_cache(RelativeEpoch::Previous, spec)?;
+        self.build_epoch_cache(RelativeEpoch::Current, spec)?;
+        self.build_epoch_cache(RelativeEpoch::Next, spec)?;
+
+        /*
+         * Validators attesting during the current epoch.
+         */
+        let active_validator_indices = get_active_validator_indices(
+            &self.validator_registry,
+            self.slot.epoch(spec.epoch_length),
+        );
+        let current_total_balance = self.get_total_balance(&active_validator_indices[..], spec);
+
+        trace!(
+            "{} validators with a total balance of {} wei.",
+            active_validator_indices.len(),
+            current_total_balance
+        );
+
+        let current_epoch_attestations: Vec<&PendingAttestation> = self
+            .latest_attestations
+            .par_iter()
+            .filter(|a| {
+                (a.data.slot / spec.epoch_length).epoch(spec.epoch_length)
+                    == self.current_epoch(spec)
+            })
+            .collect();
+
+        trace!(
+            "Current epoch attestations: {}",
+            current_epoch_attestations.len()
+        );
+
+        let current_epoch_boundary_attestations: Vec<&PendingAttestation> =
+            current_epoch_attestations
+                .par_iter()
+                .filter(
+                    |a| match self.get_block_root(self.current_epoch_start_slot(spec), spec) {
+                        Some(block_root) => {
+                            (a.data.epoch_boundary_root == *block_root)
+                                && (a.data.justified_epoch == self.justified_epoch)
+                        }
+                        None => unreachable!(),
+                    },
+                )
+                .cloned()
+                .collect();
+
+        let current_epoch_boundary_attester_indices = self
+            .get_attestation_participants_union(&current_epoch_boundary_attestations[..], spec)?;
+        let current_epoch_boundary_attesting_balance =
+            self.get_total_balance(&current_epoch_boundary_attester_indices[..], spec);
+
+        trace!(
+            "Current epoch boundary attesters: {}",
+            current_epoch_boundary_attester_indices.len()
+        );
+
+        /*
+         * Validators attesting during the previous epoch
+         */
+
+        /*
+         * Validators that made an attestation during the previous epoch
+         */
+        let previous_epoch_attestations: Vec<&PendingAttestation> = self
+            .latest_attestations
+            .par_iter()
+            .filter(|a| {
+                //TODO: ensure these saturating subs are correct.
+                (a.data.slot / spec.epoch_length).epoch(spec.epoch_length)
+                    == self.previous_epoch(spec)
+            })
+            .collect();
+
+        debug!(
+            "previous epoch attestations: {}",
+            previous_epoch_attestations.len()
+        );
+
+        let previous_epoch_attester_indices =
+            self.get_attestation_participants_union(&previous_epoch_attestations[..], spec)?;
+        let previous_total_balance = self.get_total_balance(
+            &get_active_validator_indices(&self.validator_registry, previous_epoch),
+            spec,
+        );
+
+        /*
+         * Validators targetting the previous justified slot
+         */
+        let previous_epoch_justified_attestations: Vec<&PendingAttestation> = {
+            let mut a: Vec<&PendingAttestation> = current_epoch_attestations
+                .iter()
+                .filter(|a| a.data.justified_epoch == self.previous_justified_epoch)
+                .cloned()
+                .collect();
+            let mut b: Vec<&PendingAttestation> = previous_epoch_attestations
+                .iter()
+                .filter(|a| a.data.justified_epoch == self.previous_justified_epoch)
+                .cloned()
+                .collect();
+            a.append(&mut b);
+            a
+        };
+
+        let previous_epoch_justified_attester_indices = self
+            .get_attestation_participants_union(&previous_epoch_justified_attestations[..], spec)?;
+        let previous_epoch_justified_attesting_balance =
+            self.get_total_balance(&previous_epoch_justified_attester_indices[..], spec);
+
+        /*
+         * Validators justifying the epoch boundary block at the start of the previous epoch
+         */
+        let previous_epoch_boundary_attestations: Vec<&PendingAttestation> =
+            previous_epoch_justified_attestations
+                .iter()
+                .filter(
+                    |a| match self.get_block_root(self.previous_epoch_start_slot(spec), spec) {
+                        Some(block_root) => a.data.epoch_boundary_root == *block_root,
+                        None => unreachable!(),
+                    },
+                )
+                .cloned()
+                .collect();
+
+        let previous_epoch_boundary_attester_indices = self
+            .get_attestation_participants_union(&previous_epoch_boundary_attestations[..], spec)?;
+        let previous_epoch_boundary_attesting_balance =
+            self.get_total_balance(&previous_epoch_boundary_attester_indices[..], spec);
+
+        /*
+         * Validators attesting to the expected beacon chain head during the previous epoch.
+         */
+        let previous_epoch_head_attestations: Vec<&PendingAttestation> =
+            previous_epoch_attestations
+                .iter()
+                .filter(|a| match self.get_block_root(a.data.slot, spec) {
+                    Some(block_root) => a.data.beacon_block_root == *block_root,
+                    None => unreachable!(),
+                })
+                .cloned()
+                .collect();
+
+        let previous_epoch_head_attester_indices =
+            self.get_attestation_participants_union(&previous_epoch_head_attestations[..], spec)?;
+        let previous_epoch_head_attesting_balance =
+            self.get_total_balance(&previous_epoch_head_attester_indices[..], spec);
+
+        debug!(
+            "previous_epoch_head_attester_balance of {} wei.",
+            previous_epoch_head_attesting_balance
+        );
+
+        /*
+         * Eth1 Data
+         */
+        if self.next_epoch(spec) % spec.eth1_data_voting_period == 0 {
+            for eth1_data_vote in &self.eth1_data_votes {
+                if eth1_data_vote.vote_count * 2 > spec.eth1_data_voting_period {
+                    self.latest_eth1_data = eth1_data_vote.eth1_data.clone();
+                }
+            }
+            self.eth1_data_votes = vec![];
+        }
+
+        /*
+         * Justification
+         */
+
+        let mut new_justified_epoch = self.justified_epoch;
+        self.justification_bitfield <<= 1;
+
+        // If > 2/3 of the total balance attested to the previous epoch boundary
+        //
+        // - Set the 2nd bit of the bitfield.
+        // - Set the previous epoch to be justified.
+        if (3 * previous_epoch_boundary_attesting_balance) >= (2 * current_total_balance) {
+            self.justification_bitfield |= 2;
+            new_justified_epoch = previous_epoch;
+            trace!(">= 2/3 voted for previous epoch boundary");
+        }
+        // If > 2/3 of the total balance attested to the previous epoch boundary
+        //
+        // - Set the 1st bit of the bitfield.
+        // - Set the current epoch to be justified.
+        if (3 * current_epoch_boundary_attesting_balance) >= (2 * current_total_balance) {
+            self.justification_bitfield |= 1;
+            new_justified_epoch = current_epoch;
+            trace!(">= 2/3 voted for current epoch boundary");
+        }
+
+        // If:
+        //
+        // - All three epochs prior to this epoch have been justified.
+        // - The previous justified justified epoch was three epochs ago.
+        //
+        // Then, set the finalized epoch to be three epochs ago.
+        if ((self.justification_bitfield >> 1) % 8 == 0b111)
+            & (self.previous_justified_epoch == previous_epoch - 2)
+        {
+            self.finalized_epoch = self.previous_justified_epoch;
+            trace!("epoch - 3 was finalized (1st condition).");
+        }
+        // If:
+        //
+        // - Both two epochs prior to this epoch have been justified.
+        // - The previous justified epoch was two epochs ago.
+        //
+        // Then, set the finalized epoch to two epochs ago.
+        if ((self.justification_bitfield >> 1) % 4 == 0b11)
+            & (self.previous_justified_epoch == previous_epoch - 1)
+        {
+            self.finalized_epoch = self.previous_justified_epoch;
+            trace!("epoch - 2 was finalized (2nd condition).");
+        }
+        // If:
+        //
+        // - This epoch and the two prior have been justified.
+        // - The presently justified epoch was two epochs ago.
+        //
+        // Then, set the finalized epoch to two epochs ago.
+        if (self.justification_bitfield % 8 == 0b111) & (self.justified_epoch == previous_epoch - 1)
+        {
+            self.finalized_epoch = self.justified_epoch;
+            trace!("epoch - 2 was finalized (3rd condition).");
+        }
+        // If:
+        //
+        // - This epoch and the epoch prior to it have been justified.
+        // - Set the previous epoch to be justified.
+        //
+        // Then, set the finalized epoch to be the previous epoch.
+        if (self.justification_bitfield % 4 == 0b11) & (self.justified_epoch == previous_epoch) {
+            self.finalized_epoch = self.justified_epoch;
+            trace!("epoch - 1 was finalized (4th condition).");
+        }
+
+        self.previous_justified_epoch = self.justified_epoch;
+        self.justified_epoch = new_justified_epoch;
+
+        debug!(
+            "Finalized epoch {}, justified epoch {}.",
+            self.finalized_epoch, self.justified_epoch
+        );
+
+        /*
+         * Crosslinks
+         */
+
+        // Cached for later lookups.
+        let mut winning_root_for_shards: HashMap<u64, Result<WinningRoot, WinningRootError>> =
+            HashMap::new();
+
+        // for slot in self.slot.saturating_sub(2 * spec.epoch_length)..self.slot {
+        for slot in self.previous_epoch(spec).slot_iter(spec.epoch_length) {
+            trace!(
+                "Finding winning root for slot: {} (epoch: {})",
+                slot,
+                slot.epoch(spec.epoch_length)
+            );
+
+            // Clone is used to remove the borrow. It becomes an issue later when trying to mutate
+            // `self.balances`.
+            let crosslink_committees_at_slot =
+                self.get_crosslink_committees_at_slot(slot, spec)?.clone();
+
+            for (crosslink_committee, shard) in crosslink_committees_at_slot {
+                let shard = shard as u64;
+
+                let winning_root = winning_root(
+                    self,
+                    shard,
+                    &current_epoch_attestations,
+                    &previous_epoch_attestations,
+                    spec,
+                );
+
+                if let Ok(winning_root) = &winning_root {
+                    let total_committee_balance =
+                        self.get_total_balance(&crosslink_committee[..], spec);
+
+                    if (3 * winning_root.total_attesting_balance) >= (2 * total_committee_balance) {
+                        self.latest_crosslinks[shard as usize] = Crosslink {
+                            epoch: current_epoch,
+                            shard_block_root: winning_root.shard_block_root,
+                        }
+                    }
+                }
+                winning_root_for_shards.insert(shard, winning_root);
+            }
+        }
+
+        trace!(
+            "Found {} winning shard roots.",
+            winning_root_for_shards.len()
+        );
+
+        /*
+         * Rewards and Penalities
+         */
+        let base_reward_quotient =
+            previous_total_balance.integer_sqrt() / spec.base_reward_quotient;
+        if base_reward_quotient == 0 {
+            return Err(Error::BaseRewardQuotientIsZero);
+        }
+
+        /*
+         * Justification and finalization
+         */
+        let epochs_since_finality = next_epoch - self.finalized_epoch;
+
+        let previous_epoch_justified_attester_indices_hashset: HashSet<usize> =
+            HashSet::from_iter(previous_epoch_justified_attester_indices.iter().cloned());
+        let previous_epoch_boundary_attester_indices_hashset: HashSet<usize> =
+            HashSet::from_iter(previous_epoch_boundary_attester_indices.iter().cloned());
+        let previous_epoch_head_attester_indices_hashset: HashSet<usize> =
+            HashSet::from_iter(previous_epoch_head_attester_indices.iter().cloned());
+        let previous_epoch_attester_indices_hashset: HashSet<usize> =
+            HashSet::from_iter(previous_epoch_attester_indices.iter().cloned());
+        let active_validator_indices_hashset: HashSet<usize> =
+            HashSet::from_iter(active_validator_indices.iter().cloned());
+
+        debug!("previous epoch justified attesters: {}, previous epoch boundary attesters: {}, previous epoch head attesters: {}, previous epoch attesters: {}", previous_epoch_justified_attester_indices.len(), previous_epoch_boundary_attester_indices.len(), previous_epoch_head_attester_indices.len(), previous_epoch_attester_indices.len());
+
+        debug!("{} epochs since finality.", epochs_since_finality);
+
+        if epochs_since_finality <= 4 {
+            for index in 0..self.validator_balances.len() {
+                let base_reward = self.base_reward(index, base_reward_quotient, spec);
+
+                if previous_epoch_justified_attester_indices_hashset.contains(&index) {
+                    safe_add_assign!(
+                        self.validator_balances[index],
+                        base_reward * previous_epoch_justified_attesting_balance
+                            / previous_total_balance
+                    );
+                } else if active_validator_indices_hashset.contains(&index) {
+                    safe_sub_assign!(self.validator_balances[index], base_reward);
+                }
+
+                if previous_epoch_boundary_attester_indices_hashset.contains(&index) {
+                    safe_add_assign!(
+                        self.validator_balances[index],
+                        base_reward * previous_epoch_boundary_attesting_balance
+                            / previous_total_balance
+                    );
+                } else if active_validator_indices_hashset.contains(&index) {
+                    safe_sub_assign!(self.validator_balances[index], base_reward);
+                }
+
+                if previous_epoch_head_attester_indices_hashset.contains(&index) {
+                    safe_add_assign!(
+                        self.validator_balances[index],
+                        base_reward * previous_epoch_head_attesting_balance
+                            / previous_total_balance
+                    );
+                } else if active_validator_indices_hashset.contains(&index) {
+                    safe_sub_assign!(self.validator_balances[index], base_reward);
+                }
+            }
+
+            for index in previous_epoch_attester_indices {
+                let base_reward = self.base_reward(index, base_reward_quotient, spec);
+                let inclusion_distance =
+                    self.inclusion_distance(&previous_epoch_attestations, index, spec)?;
+
+                safe_add_assign!(
+                    self.validator_balances[index],
+                    base_reward * spec.min_attestation_inclusion_delay / inclusion_distance
+                )
+            }
+        } else {
+            for index in 0..self.validator_balances.len() {
+                let inactivity_penalty = self.inactivity_penalty(
+                    index,
+                    epochs_since_finality,
+                    base_reward_quotient,
+                    spec,
+                );
+                if active_validator_indices_hashset.contains(&index) {
+                    if !previous_epoch_justified_attester_indices_hashset.contains(&index) {
+                        safe_sub_assign!(self.validator_balances[index], inactivity_penalty);
+                    }
+                    if !previous_epoch_boundary_attester_indices_hashset.contains(&index) {
+                        safe_sub_assign!(self.validator_balances[index], inactivity_penalty);
+                    }
+                    if !previous_epoch_head_attester_indices_hashset.contains(&index) {
+                        safe_sub_assign!(self.validator_balances[index], inactivity_penalty);
+                    }
+
+                    if self.validator_registry[index].penalized_epoch <= current_epoch {
+                        let base_reward = self.base_reward(index, base_reward_quotient, spec);
+                        safe_sub_assign!(
+                            self.validator_balances[index],
+                            2 * inactivity_penalty + base_reward
+                        );
+                    }
+                }
+            }
+
+            for index in previous_epoch_attester_indices {
+                let base_reward = self.base_reward(index, base_reward_quotient, spec);
+                let inclusion_distance =
+                    self.inclusion_distance(&previous_epoch_attestations, index, spec)?;
+
+                safe_sub_assign!(
+                    self.validator_balances[index],
+                    base_reward
+                        - base_reward * spec.min_attestation_inclusion_delay / inclusion_distance
+                );
+            }
+        }
+
+        trace!("Processed validator justification and finalization rewards/penalities.");
+
+        /*
+         * Attestation inclusion
+         */
+        for &index in &previous_epoch_attester_indices_hashset {
+            let inclusion_slot =
+                self.inclusion_slot(&previous_epoch_attestations[..], index, spec)?;
+            let proposer_index = self
+                .get_beacon_proposer_index(inclusion_slot, spec)
+                .map_err(|_| Error::UnableToDetermineProducer)?;
+            let base_reward = self.base_reward(proposer_index, base_reward_quotient, spec);
+            safe_add_assign!(
+                self.validator_balances[proposer_index],
+                base_reward / spec.includer_reward_quotient
+            );
+        }
+
+        trace!(
+            "Previous epoch attesters: {}.",
+            previous_epoch_attester_indices_hashset.len()
+        );
+
+        /*
+         * Crosslinks
+         */
+        for slot in self.previous_epoch(spec).slot_iter(spec.epoch_length) {
+            // Clone is used to remove the borrow. It becomes an issue later when trying to mutate
+            // `self.balances`.
+            let crosslink_committees_at_slot =
+                self.get_crosslink_committees_at_slot(slot, spec)?.clone();
+
+            for (_crosslink_committee, shard) in crosslink_committees_at_slot {
+                let shard = shard as u64;
+
+                if let Some(Ok(winning_root)) = winning_root_for_shards.get(&shard) {
+                    // TODO: remove the map.
+                    let attesting_validator_indices: HashSet<usize> = HashSet::from_iter(
+                        winning_root.attesting_validator_indices.iter().cloned(),
+                    );
+
+                    for index in 0..self.validator_balances.len() {
+                        let base_reward = self.base_reward(index, base_reward_quotient, spec);
+
+                        if attesting_validator_indices.contains(&index) {
+                            safe_add_assign!(
+                                self.validator_balances[index],
+                                base_reward * winning_root.total_attesting_balance
+                                    / winning_root.total_balance
+                            );
+                        } else {
+                            safe_sub_assign!(self.validator_balances[index], base_reward);
+                        }
+                    }
+
+                    for index in &winning_root.attesting_validator_indices {
+                        let base_reward = self.base_reward(*index, base_reward_quotient, spec);
+                        safe_add_assign!(
+                            self.validator_balances[*index],
+                            base_reward * winning_root.total_attesting_balance
+                                / winning_root.total_balance
+                        );
+                    }
+                }
+            }
+        }
+
+        /*
+         * Ejections
+         */
+        self.process_ejections(spec);
+
+        /*
+         * Validator Registry
+         */
+        self.previous_calculation_epoch = self.current_calculation_epoch;
+        self.previous_epoch_start_shard = self.current_epoch_start_shard;
+
+        debug!(
+            "setting previous_epoch_seed to : {}",
+            self.current_epoch_seed
+        );
+
+        self.previous_epoch_seed = self.current_epoch_seed;
+
+        let should_update_validator_registy = if self.finalized_epoch
+            > self.validator_registry_update_epoch
+        {
+            (0..self.get_current_epoch_committee_count(spec)).all(|i| {
+                let shard = (self.current_epoch_start_shard + i as u64) % spec.shard_count;
+                self.latest_crosslinks[shard as usize].epoch > self.validator_registry_update_epoch
+            })
+        } else {
+            false
+        };
+
+        if should_update_validator_registy {
+            trace!("updating validator registry.");
+            self.update_validator_registry(spec);
+
+            self.current_calculation_epoch = next_epoch;
+            self.current_epoch_start_shard = (self.current_epoch_start_shard
+                + self.get_current_epoch_committee_count(spec) as u64)
+                % spec.shard_count;
+            self.current_epoch_seed = self.generate_seed(self.current_calculation_epoch, spec)?
+        } else {
+            trace!("not updating validator registry.");
+            let epochs_since_last_registry_update =
+                current_epoch - self.validator_registry_update_epoch;
+            if (epochs_since_last_registry_update > 1)
+                & epochs_since_last_registry_update.is_power_of_two()
+            {
+                self.current_calculation_epoch = next_epoch;
+                self.current_epoch_seed =
+                    self.generate_seed(self.current_calculation_epoch, spec)?
+            }
+        }
+
+        self.process_penalties_and_exits(spec);
+
+        self.latest_index_roots[(next_epoch.as_usize() + spec.entry_exit_delay as usize)
+            % spec.latest_index_roots_length] = hash_tree_root(get_active_validator_indices(
+            &self.validator_registry,
+            next_epoch + Epoch::from(spec.entry_exit_delay),
+        ));
+        self.latest_penalized_balances[next_epoch.as_usize() % spec.latest_penalized_exit_length] =
+            self.latest_penalized_balances
+                [current_epoch.as_usize() % spec.latest_penalized_exit_length];
+        self.latest_randao_mixes[next_epoch.as_usize() % spec.latest_randao_mixes_length] = self
+            .get_randao_mix(current_epoch, spec)
+            .and_then(|x| Some(*x))
+            .ok_or_else(|| Error::NoRandaoSeed)?;
+        self.latest_attestations = self
+            .latest_attestations
+            .iter()
+            .filter(|a| a.data.slot.epoch(spec.epoch_length) >= current_epoch)
+            .cloned()
+            .collect();
+
+        debug!("Epoch transition complete.");
+
+        Ok(())
+    }
+}
+
+fn hash_tree_root<T: TreeHash>(input: Vec<T>) -> Hash256 {
+    Hash256::from_slice(&input.hash_tree_root()[..])
+}
+
+fn winning_root(
+    state: &BeaconState,
+    shard: u64,
+    current_epoch_attestations: &[&PendingAttestation],
+    previous_epoch_attestations: &[&PendingAttestation],
+    spec: &ChainSpec,
+) -> Result<WinningRoot, WinningRootError> {
+    let mut attestations = current_epoch_attestations.to_vec();
+    attestations.append(&mut previous_epoch_attestations.to_vec());
+
+    let mut candidates: HashMap<Hash256, WinningRoot> = HashMap::new();
+
+    let mut highest_seen_balance = 0;
+
+    for a in &attestations {
+        if a.data.shard != shard {
+            continue;
+        }
+
+        let shard_block_root = &a.data.shard_block_root;
+
+        if candidates.contains_key(shard_block_root) {
+            continue;
+        }
+
+        let attesting_validator_indices = attestations
+            .iter()
+            .try_fold::<_, _, Result<_, BeaconStateError>>(vec![], |mut acc, a| {
+                if (a.data.shard == shard) && (a.data.shard_block_root == *shard_block_root) {
+                    acc.append(&mut state.get_attestation_participants(
+                        &a.data,
+                        &a.aggregation_bitfield,
+                        spec,
+                    )?);
+                }
+                Ok(acc)
+            })?;
+
+        let total_balance: u64 = attesting_validator_indices
+            .iter()
+            .fold(0, |acc, i| acc + state.get_effective_balance(*i, spec));
+
+        let total_attesting_balance: u64 = attesting_validator_indices
+            .iter()
+            .fold(0, |acc, i| acc + state.get_effective_balance(*i, spec));
+
+        if total_attesting_balance > highest_seen_balance {
+            highest_seen_balance = total_attesting_balance;
+        }
+
+        let candidate_root = WinningRoot {
+            shard_block_root: *shard_block_root,
+            attesting_validator_indices,
+            total_attesting_balance,
+            total_balance,
+        };
+
+        candidates.insert(*shard_block_root, candidate_root);
+    }
+
+    Ok(candidates
+        .iter()
+        .filter_map(|(_hash, candidate)| {
+            if candidate.total_attesting_balance == highest_seen_balance {
+                Some(candidate)
+            } else {
+                None
+            }
+        })
+        .min_by_key(|candidate| candidate.shard_block_root)
+        .ok_or_else(|| WinningRootError::NoWinningRoot)?
+        // TODO: avoid clone.
+        .clone())
+}
+
+impl From<InclusionError> for Error {
+    fn from(e: InclusionError) -> Error {
+        Error::InclusionError(e)
+    }
+}
+
+impl From<BeaconStateError> for Error {
+    fn from(e: BeaconStateError) -> Error {
+        Error::BeaconStateError(e)
+    }
+}
+
+impl From<BeaconStateError> for WinningRootError {
+    fn from(e: BeaconStateError) -> WinningRootError {
+        WinningRootError::BeaconStateError(e)
+    }
+}
diff --git a/eth2/state_processing/src/per_block_processing.rs b/eth2/state_processing/src/per_block_processing.rs
index fc3a8204e..06a5f81c7 100644
--- a/eth2/state_processing/src/per_block_processing.rs
+++ b/eth2/state_processing/src/per_block_processing.rs
@@ -99,7 +99,7 @@ pub fn verify_block_signature(
     let proposal = Proposal {
         slot: block.slot,
         shard: spec.beacon_chain_shard_number,
-        block_root: Hash256::from(&block.signed_root()[..]),
+        block_root: Hash256::from_slice(&block.signed_root()[..]),
         signature: block.signature.clone(),
     };
     let domain = spec.get_domain(
@@ -187,7 +187,7 @@ pub fn update_randao(
 ) -> Result<(), BeaconStateError> {
     let hashed_reveal = {
         let encoded_signature = ssz_encode(reveal);
-        Hash256::from(&hash(&encoded_signature[..])[..])
+        Hash256::from_slice(&hash(&encoded_signature[..])[..])
     };
 
     let current_epoch = state.slot.epoch(spec.slots_per_epoch);
diff --git a/eth2/state_processing/src/per_epoch_processing.rs b/eth2/state_processing/src/per_epoch_processing.rs
index 732a6e48f..bd8aca3c4 100644
--- a/eth2/state_processing/src/per_epoch_processing.rs
+++ b/eth2/state_processing/src/per_epoch_processing.rs
@@ -84,7 +84,7 @@ pub fn per_epoch_processing(state: &mut BeaconState, spec: &ChainSpec) -> Result
     .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(&active_tree_root[..]);
+        % 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];
diff --git a/eth2/types/Cargo.toml b/eth2/types/Cargo.toml
index f611aa3c1..ea1343dba 100644
--- a/eth2/types/Cargo.toml
+++ b/eth2/types/Cargo.toml
@@ -7,7 +7,7 @@ edition = "2018"
 [dependencies]
 bls = { path = "../utils/bls" }
 boolean-bitfield = { path = "../utils/boolean-bitfield" }
-ethereum-types = "0.4.0"
+ethereum-types = "0.5"
 hashing = { path = "../utils/hashing" }
 honey-badger-split =  { path = "../utils/honey-badger-split" }
 int_to_bytes = { path = "../utils/int_to_bytes" }
diff --git a/eth2/types/src/attester_slashing/builder.rs b/eth2/types/src/attester_slashing/builder.rs
index bdb46b154..46dcaf174 100644
--- a/eth2/types/src/attester_slashing/builder.rs
+++ b/eth2/types/src/attester_slashing/builder.rs
@@ -27,8 +27,8 @@ impl AttesterSlashingBuilder {
         let shard = 0;
         let justified_epoch = Epoch::new(0);
         let epoch = Epoch::new(0);
-        let hash_1 = Hash256::from(&[1][..]);
-        let hash_2 = Hash256::from(&[2][..]);
+        let hash_1 = Hash256::from_low_u64_le(1);
+        let hash_2 = Hash256::from_low_u64_le(2);
 
         let mut slashable_attestation_1 = SlashableAttestation {
             validator_indices: validator_indices.to_vec(),
diff --git a/eth2/types/src/beacon_state.rs b/eth2/types/src/beacon_state.rs
index bb77981ab..ac8cf164d 100644
--- a/eth2/types/src/beacon_state.rs
+++ b/eth2/types/src/beacon_state.rs
@@ -469,18 +469,20 @@ impl BeaconState {
         let mut input = self
             .get_randao_mix(epoch - spec.min_seed_lookahead, spec)
             .ok_or_else(|| Error::InsufficientRandaoMixes)?
+            .as_bytes()
             .to_vec();
 
         input.append(
             &mut self
                 .get_active_index_root(epoch, spec)
                 .ok_or_else(|| Error::InsufficientIndexRoots)?
+                .as_bytes()
                 .to_vec(),
         );
 
         input.append(&mut int_to_bytes32(epoch.as_u64()));
 
-        Ok(Hash256::from(&hash(&input[..])[..]))
+        Ok(Hash256::from_slice(&hash(&input[..])[..]))
     }
 
     /// Returns the beacon proposer index for the `slot`.
@@ -1155,7 +1157,7 @@ impl BeaconState {
 }
 
 fn hash_tree_root<T: TreeHash>(input: Vec<T>) -> Hash256 {
-    Hash256::from(&input.hash_tree_root()[..])
+    Hash256::from_slice(&input.hash_tree_root()[..])
 }
 
 impl Encodable for BeaconState {
diff --git a/eth2/types/src/beacon_state/builder.rs b/eth2/types/src/beacon_state/builder.rs
index dadce9cac..0be297db7 100644
--- a/eth2/types/src/beacon_state/builder.rs
+++ b/eth2/types/src/beacon_state/builder.rs
@@ -145,8 +145,8 @@ impl BeaconStateBuilder {
         state.previous_shuffling_epoch = epoch - 1;
         state.current_shuffling_epoch = epoch;
 
-        state.previous_shuffling_seed = Hash256::from(&b"previous_seed"[..]);
-        state.current_shuffling_seed = Hash256::from(&b"current_seed"[..]);
+        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;
diff --git a/eth2/types/src/proposal_signed_data.rs b/eth2/types/src/proposal_signed_data.rs
new file mode 100644
index 000000000..58f45a41d
--- /dev/null
+++ b/eth2/types/src/proposal_signed_data.rs
@@ -0,0 +1,20 @@
+use crate::test_utils::TestRandom;
+use crate::{Hash256, Slot};
+use rand::RngCore;
+use serde_derive::Serialize;
+use ssz_derive::{Decode, Encode, TreeHash};
+use test_random_derive::TestRandom;
+
+#[derive(Debug, PartialEq, Clone, Default, Serialize, Encode, Decode, TreeHash, TestRandom)]
+pub struct ProposalSignedData {
+    pub slot: Slot,
+    pub shard: u64,
+    pub block_root: Hash256,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    ssz_tests!(ProposalSignedData);
+}
diff --git a/eth2/types/src/proposer_slashing/builder.rs b/eth2/types/src/proposer_slashing/builder.rs
index 1110bdf30..472a76ec1 100644
--- a/eth2/types/src/proposer_slashing/builder.rs
+++ b/eth2/types/src/proposer_slashing/builder.rs
@@ -25,14 +25,14 @@ impl ProposerSlashingBuilder {
         let mut proposal_1 = Proposal {
             slot,
             shard,
-            block_root: Hash256::from(&[1][..]),
+            block_root: Hash256::from_low_u64_le(1),
             signature: Signature::empty_signature(),
         };
 
         let mut proposal_2 = Proposal {
             slot,
             shard,
-            block_root: Hash256::from(&[2][..]),
+            block_root: Hash256::from_low_u64_le(2),
             signature: Signature::empty_signature(),
         };
 
diff --git a/eth2/types/src/slashable_vote_data.rs b/eth2/types/src/slashable_vote_data.rs
new file mode 100644
index 000000000..73cf91c61
--- /dev/null
+++ b/eth2/types/src/slashable_vote_data.rs
@@ -0,0 +1,132 @@
+use super::AttestationData;
+use crate::chain_spec::ChainSpec;
+use crate::test_utils::TestRandom;
+use bls::AggregateSignature;
+use rand::RngCore;
+use serde_derive::Serialize;
+use ssz_derive::{Decode, Encode, TreeHash};
+use test_random_derive::TestRandom;
+
+#[derive(Debug, PartialEq, Clone, Serialize, Encode, Decode, TreeHash, TestRandom)]
+pub struct SlashableVoteData {
+    pub custody_bit_0_indices: Vec<u32>,
+    pub custody_bit_1_indices: Vec<u32>,
+    pub data: AttestationData,
+    pub aggregate_signature: AggregateSignature,
+}
+
+impl SlashableVoteData {
+    /// Check if ``attestation_data_1`` and ``attestation_data_2`` have the same target.
+    ///
+    /// Spec v0.3.0
+    pub fn is_double_vote(&self, other: &SlashableVoteData, spec: &ChainSpec) -> bool {
+        self.data.slot.epoch(spec.epoch_length) == other.data.slot.epoch(spec.epoch_length)
+    }
+
+    /// Check if ``attestation_data_1`` surrounds ``attestation_data_2``.
+    ///
+    /// Spec v0.3.0
+    pub fn is_surround_vote(&self, other: &SlashableVoteData, spec: &ChainSpec) -> bool {
+        let source_epoch_1 = self.data.justified_epoch;
+        let source_epoch_2 = other.data.justified_epoch;
+        let target_epoch_1 = self.data.slot.epoch(spec.epoch_length);
+        let target_epoch_2 = other.data.slot.epoch(spec.epoch_length);
+
+        (source_epoch_1 < source_epoch_2) && (target_epoch_2 < target_epoch_1)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::chain_spec::ChainSpec;
+    use crate::slot_epoch::{Epoch, Slot};
+    use crate::test_utils::{SeedableRng, TestRandom, XorShiftRng};
+
+    #[test]
+    pub fn test_is_double_vote_true() {
+        let spec = ChainSpec::foundation();
+        let slashable_vote_first = create_slashable_vote_data(1, 1, &spec);
+        let slashable_vote_second = create_slashable_vote_data(1, 1, &spec);
+
+        assert_eq!(
+            slashable_vote_first.is_double_vote(&slashable_vote_second, &spec),
+            true
+        )
+    }
+
+    #[test]
+    pub fn test_is_double_vote_false() {
+        let spec = ChainSpec::foundation();
+        let slashable_vote_first = create_slashable_vote_data(1, 1, &spec);
+        let slashable_vote_second = create_slashable_vote_data(2, 1, &spec);
+
+        assert_eq!(
+            slashable_vote_first.is_double_vote(&slashable_vote_second, &spec),
+            false
+        );
+    }
+
+    #[test]
+    pub fn test_is_surround_vote_true() {
+        let spec = ChainSpec::foundation();
+        let slashable_vote_first = create_slashable_vote_data(2, 1, &spec);
+        let slashable_vote_second = create_slashable_vote_data(1, 2, &spec);
+
+        assert_eq!(
+            slashable_vote_first.is_surround_vote(&slashable_vote_second, &spec),
+            true
+        );
+    }
+
+    #[test]
+    pub fn test_is_surround_vote_true_realistic() {
+        let spec = ChainSpec::foundation();
+        let slashable_vote_first = create_slashable_vote_data(4, 1, &spec);
+        let slashable_vote_second = create_slashable_vote_data(3, 2, &spec);
+
+        assert_eq!(
+            slashable_vote_first.is_surround_vote(&slashable_vote_second, &spec),
+            true
+        );
+    }
+
+    #[test]
+    pub fn test_is_surround_vote_false_source_epoch_fails() {
+        let spec = ChainSpec::foundation();
+        let slashable_vote_first = create_slashable_vote_data(2, 2, &spec);
+        let slashable_vote_second = create_slashable_vote_data(1, 1, &spec);
+
+        assert_eq!(
+            slashable_vote_first.is_surround_vote(&slashable_vote_second, &spec),
+            false
+        );
+    }
+
+    #[test]
+    pub fn test_is_surround_vote_false_target_epoch_fails() {
+        let spec = ChainSpec::foundation();
+        let slashable_vote_first = create_slashable_vote_data(1, 1, &spec);
+        let slashable_vote_second = create_slashable_vote_data(2, 2, &spec);
+
+        assert_eq!(
+            slashable_vote_first.is_surround_vote(&slashable_vote_second, &spec),
+            false
+        );
+    }
+
+    ssz_tests!(SlashableVoteData);
+
+    fn create_slashable_vote_data(
+        slot_factor: u64,
+        justified_epoch: u64,
+        spec: &ChainSpec,
+    ) -> SlashableVoteData {
+        let mut rng = XorShiftRng::from_seed([42; 16]);
+        let mut slashable_vote = SlashableVoteData::random_for_test(&mut rng);
+
+        slashable_vote.data.slot = Slot::new(slot_factor * spec.epoch_length);
+        slashable_vote.data.justified_epoch = Epoch::new(justified_epoch);
+        slashable_vote
+    }
+}
diff --git a/eth2/types/src/test_utils/address.rs b/eth2/types/src/test_utils/address.rs
index 2d60b72da..13de2dec9 100644
--- a/eth2/types/src/test_utils/address.rs
+++ b/eth2/types/src/test_utils/address.rs
@@ -6,6 +6,6 @@ impl<T: RngCore> TestRandom<T> for Address {
     fn random_for_test(rng: &mut T) -> Self {
         let mut key_bytes = vec![0; 20];
         rng.fill_bytes(&mut key_bytes);
-        Address::from(&key_bytes[..])
+        Address::from_slice(&key_bytes[..])
     }
 }
diff --git a/eth2/types/src/test_utils/hash256.rs b/eth2/types/src/test_utils/hash256.rs
index 98f5e7899..a227679da 100644
--- a/eth2/types/src/test_utils/hash256.rs
+++ b/eth2/types/src/test_utils/hash256.rs
@@ -6,6 +6,6 @@ impl<T: RngCore> TestRandom<T> for Hash256 {
     fn random_for_test(rng: &mut T) -> Self {
         let mut key_bytes = vec![0; 32];
         rng.fill_bytes(&mut key_bytes);
-        Hash256::from(&key_bytes[..])
+        Hash256::from_slice(&key_bytes[..])
     }
 }
diff --git a/eth2/types/src/test_utils/macros.rs b/eth2/types/src/test_utils/macros.rs
new file mode 100644
index 000000000..b7c0a6522
--- /dev/null
+++ b/eth2/types/src/test_utils/macros.rs
@@ -0,0 +1,34 @@
+#[cfg(test)]
+#[macro_export]
+macro_rules! ssz_tests {
+    ($type: ident) => {
+        #[test]
+        pub fn test_ssz_round_trip() {
+            use crate::test_utils::{SeedableRng, TestRandom, XorShiftRng};
+            use ssz::{ssz_encode, Decodable};
+
+            let mut rng = XorShiftRng::from_seed([42; 16]);
+            let original = $type::random_for_test(&mut rng);
+
+            let bytes = ssz_encode(&original);
+            let (decoded, _) = $type::ssz_decode(&bytes, 0).unwrap();
+
+            assert_eq!(original, decoded);
+        }
+
+        #[test]
+        pub fn test_hash_tree_root_internal() {
+            use crate::test_utils::{SeedableRng, TestRandom, XorShiftRng};
+            use ssz::TreeHash;
+
+            let mut rng = XorShiftRng::from_seed([42; 16]);
+            let original = $type::random_for_test(&mut rng);
+
+            let result = original.hash_tree_root_internal();
+
+            assert_eq!(result.len(), 32);
+            // TODO: Add further tests
+            // https://github.com/sigp/lighthouse/issues/170
+        }
+    };
+}
diff --git a/eth2/types/src/validator_registry_delta_block.rs b/eth2/types/src/validator_registry_delta_block.rs
new file mode 100644
index 000000000..e9a075052
--- /dev/null
+++ b/eth2/types/src/validator_registry_delta_block.rs
@@ -0,0 +1,36 @@
+use crate::{test_utils::TestRandom, Hash256, Slot};
+use bls::PublicKey;
+use rand::RngCore;
+use serde_derive::Serialize;
+use ssz_derive::{Decode, Encode, TreeHash};
+use test_random_derive::TestRandom;
+
+// The information gathered from the PoW chain validator registration function.
+#[derive(Debug, Clone, PartialEq, Serialize, Encode, Decode, TreeHash, TestRandom)]
+pub struct ValidatorRegistryDeltaBlock {
+    pub latest_registry_delta_root: Hash256,
+    pub validator_index: u32,
+    pub pubkey: PublicKey,
+    pub slot: Slot,
+    pub flag: u64,
+}
+
+impl Default for ValidatorRegistryDeltaBlock {
+    /// Yields a "default" `Validator`. Primarily used for testing.
+    fn default() -> Self {
+        Self {
+            latest_registry_delta_root: Hash256::zero(),
+            validator_index: std::u32::MAX,
+            pubkey: PublicKey::default(),
+            slot: Slot::from(std::u64::MAX),
+            flag: std::u64::MAX,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    ssz_tests!(ValidatorRegistryDeltaBlock);
+}
diff --git a/eth2/utils/merkle_proof/Cargo.toml b/eth2/utils/merkle_proof/Cargo.toml
new file mode 100644
index 000000000..b7cd81216
--- /dev/null
+++ b/eth2/utils/merkle_proof/Cargo.toml
@@ -0,0 +1,9 @@
+[package]
+name = "merkle_proof"
+version = "0.1.0"
+authors = ["Michael Sproul <michael@sigmaprime.io>"]
+edition = "2018"
+
+[dependencies]
+ethereum-types = "0.5"
+hashing = { path = "../hashing" }
diff --git a/eth2/utils/merkle_proof/src/lib.rs b/eth2/utils/merkle_proof/src/lib.rs
new file mode 100644
index 000000000..4d0abcea3
--- /dev/null
+++ b/eth2/utils/merkle_proof/src/lib.rs
@@ -0,0 +1,148 @@
+use ethereum_types::H256;
+use hashing::hash;
+
+/// Verify a proof that `leaf` exists at `index` in a Merkle tree rooted at `root`.
+///
+/// The `branch` argument is the main component of the proof: it should be a list of internal
+/// node hashes such that the root can be reconstructed (in bottom-up order).
+pub fn verify_merkle_proof(
+    leaf: H256,
+    branch: &[H256],
+    depth: usize,
+    index: usize,
+    root: H256,
+) -> bool {
+    if branch.len() == depth {
+        merkle_root_from_branch(leaf, branch, depth, index) == root
+    } else {
+        false
+    }
+}
+
+/// Compute a root hash from a leaf and a Merkle proof.
+fn merkle_root_from_branch(leaf: H256, branch: &[H256], depth: usize, index: usize) -> H256 {
+    assert_eq!(branch.len(), depth, "proof length should equal depth");
+
+    let mut merkle_root = leaf.as_bytes().to_vec();
+
+    for i in 0..depth {
+        let ith_bit = (index >> i) & 0x01;
+        if ith_bit == 1 {
+            let input = concat(branch[i].as_bytes().to_vec(), merkle_root);
+            merkle_root = hash(&input);
+        } else {
+            let mut input = merkle_root;
+            input.extend_from_slice(branch[i].as_bytes());
+            merkle_root = hash(&input);
+        }
+    }
+
+    H256::from_slice(&merkle_root)
+}
+
+/// Concatenate two vectors.
+fn concat(mut vec1: Vec<u8>, mut vec2: Vec<u8>) -> Vec<u8> {
+    vec1.append(&mut vec2);
+    vec1
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn hash_concat(h1: H256, h2: H256) -> H256 {
+        H256::from_slice(&hash(&concat(
+            h1.as_bytes().to_vec(),
+            h2.as_bytes().to_vec(),
+        )))
+    }
+
+    #[test]
+    fn verify_small_example() {
+        // Construct a small merkle tree manually
+        let leaf_b00 = H256::from([0xAA; 32]);
+        let leaf_b01 = H256::from([0xBB; 32]);
+        let leaf_b10 = H256::from([0xCC; 32]);
+        let leaf_b11 = H256::from([0xDD; 32]);
+
+        let node_b0x = hash_concat(leaf_b00, leaf_b01);
+        let node_b1x = hash_concat(leaf_b10, leaf_b11);
+
+        let root = hash_concat(node_b0x, node_b1x);
+
+        // Run some proofs
+        assert!(verify_merkle_proof(
+            leaf_b00,
+            &[leaf_b01, node_b1x],
+            2,
+            0b00,
+            root
+        ));
+        assert!(verify_merkle_proof(
+            leaf_b01,
+            &[leaf_b00, node_b1x],
+            2,
+            0b01,
+            root
+        ));
+        assert!(verify_merkle_proof(
+            leaf_b10,
+            &[leaf_b11, node_b0x],
+            2,
+            0b10,
+            root
+        ));
+        assert!(verify_merkle_proof(
+            leaf_b11,
+            &[leaf_b10, node_b0x],
+            2,
+            0b11,
+            root
+        ));
+        assert!(verify_merkle_proof(
+            leaf_b11,
+            &[leaf_b10],
+            1,
+            0b11,
+            node_b1x
+        ));
+
+        // Ensure that incorrect proofs fail
+        // Zero-length proof
+        assert!(!verify_merkle_proof(leaf_b01, &[], 2, 0b01, root));
+        // Proof in reverse order
+        assert!(!verify_merkle_proof(
+            leaf_b01,
+            &[node_b1x, leaf_b00],
+            2,
+            0b01,
+            root
+        ));
+        // Proof too short
+        assert!(!verify_merkle_proof(leaf_b01, &[leaf_b00], 2, 0b01, root));
+        // Wrong index
+        assert!(!verify_merkle_proof(
+            leaf_b01,
+            &[leaf_b00, node_b1x],
+            2,
+            0b10,
+            root
+        ));
+        // Wrong root
+        assert!(!verify_merkle_proof(
+            leaf_b01,
+            &[leaf_b00, node_b1x],
+            2,
+            0b01,
+            node_b1x
+        ));
+    }
+
+    #[test]
+    fn verify_zero_depth() {
+        let leaf = H256::from([0xD6; 32]);
+        let junk = H256::from([0xD7; 32]);
+        assert!(verify_merkle_proof(leaf, &[], 0, 0, leaf));
+        assert!(!verify_merkle_proof(leaf, &[], 0, 7, junk));
+    }
+}
diff --git a/eth2/utils/ssz/Cargo.toml b/eth2/utils/ssz/Cargo.toml
index 25326cb5b..f13db5def 100644
--- a/eth2/utils/ssz/Cargo.toml
+++ b/eth2/utils/ssz/Cargo.toml
@@ -6,5 +6,5 @@ edition = "2018"
 
 [dependencies]
 bytes = "0.4.9"
-ethereum-types = "0.4.0"
+ethereum-types = "0.5"
 hashing = { path = "../hashing" }
diff --git a/eth2/utils/ssz/src/impl_decode.rs b/eth2/utils/ssz/src/impl_decode.rs
index b9ca48f9b..b13cbeb5d 100644
--- a/eth2/utils/ssz/src/impl_decode.rs
+++ b/eth2/utils/ssz/src/impl_decode.rs
@@ -59,7 +59,7 @@ impl Decodable for H256 {
         if bytes.len() < 32 || bytes.len() - 32 < index {
             Err(DecodeError::TooShort)
         } else {
-            Ok((H256::from(&bytes[index..(index + 32)]), index + 32))
+            Ok((H256::from_slice(&bytes[index..(index + 32)]), index + 32))
         }
     }
 }
@@ -69,7 +69,7 @@ impl Decodable for Address {
         if bytes.len() < 20 || bytes.len() - 20 < index {
             Err(DecodeError::TooShort)
         } else {
-            Ok((Address::from(&bytes[index..(index + 20)]), index + 20))
+            Ok((Address::from_slice(&bytes[index..(index + 20)]), index + 20))
         }
     }
 }
@@ -95,7 +95,7 @@ mod tests {
          */
         let input = vec![42_u8; 32];
         let (decoded, i) = H256::ssz_decode(&input, 0).unwrap();
-        assert_eq!(decoded.to_vec(), input);
+        assert_eq!(decoded.as_bytes(), &input[..]);
         assert_eq!(i, 32);
 
         /*
@@ -104,7 +104,7 @@ mod tests {
         let mut input = vec![42_u8; 32];
         input.push(12);
         let (decoded, i) = H256::ssz_decode(&input, 0).unwrap();
-        assert_eq!(decoded.to_vec()[..], input[0..32]);
+        assert_eq!(decoded.as_bytes(), &input[0..32]);
         assert_eq!(i, 32);
 
         /*
diff --git a/eth2/utils/ssz/src/impl_encode.rs b/eth2/utils/ssz/src/impl_encode.rs
index 5f73b8483..bb1ec42d5 100644
--- a/eth2/utils/ssz/src/impl_encode.rs
+++ b/eth2/utils/ssz/src/impl_encode.rs
@@ -55,13 +55,13 @@ impl Encodable for bool {
 
 impl Encodable for H256 {
     fn ssz_append(&self, s: &mut SszStream) {
-        s.append_encoded_raw(&self.to_vec());
+        s.append_encoded_raw(self.as_bytes());
     }
 }
 
 impl Encodable for Address {
     fn ssz_append(&self, s: &mut SszStream) {
-        s.append_encoded_raw(&self.to_vec());
+        s.append_encoded_raw(self.as_bytes());
     }
 }
 
diff --git a/validator_client/src/config.rs b/validator_client/src/config.rs
index 104a4bbe6..68405ed2f 100644
--- a/validator_client/src/config.rs
+++ b/validator_client/src/config.rs
@@ -1,11 +1,13 @@
 use std::fs;
 use std::path::PathBuf;
+use types::ChainSpec;
 
 /// Stores the core configuration for this validator instance.
 #[derive(Clone)]
 pub struct ClientConfig {
     pub data_dir: PathBuf,
     pub server: String,
+    pub spec: ChainSpec,
 }
 
 const DEFAULT_LIGHTHOUSE_DIR: &str = ".lighthouse-validators";
@@ -20,6 +22,11 @@ impl ClientConfig {
         fs::create_dir_all(&data_dir)
             .unwrap_or_else(|_| panic!("Unable to create {:?}", &data_dir));
         let server = "localhost:50051".to_string();
-        Self { data_dir, server }
+        let spec = ChainSpec::foundation();
+        Self {
+            data_dir,
+            server,
+            spec,
+        }
     }
 }
diff --git a/validator_client/src/main.rs b/validator_client/src/main.rs
index b4d8ae5db..eea213a45 100644
--- a/validator_client/src/main.rs
+++ b/validator_client/src/main.rs
@@ -43,6 +43,16 @@ fn main() {
                 .help("Address to connect to BeaconNode.")
                 .takes_value(true),
         )
+        .arg(
+            Arg::with_name("spec")
+                .long("spec")
+                .value_name("spec")
+                .short("s")
+                .help("Configuration of Beacon Chain")
+                .takes_value(true)
+                .possible_values(&["foundation", "few_validators"])
+                .default_value("foundation"),
+        )
         .get_matches();
 
     let mut config = ClientConfig::default();
@@ -62,6 +72,17 @@ fn main() {
         }
     }
 
+    // TODO: Permit loading a custom spec from file.
+    // Custom spec
+    if let Some(spec_str) = matches.value_of("spec") {
+        match spec_str {
+            "foundation" => config.spec = ChainSpec::foundation(),
+            "few_validators" => config.spec = ChainSpec::few_validators(),
+            // Should be impossible due to clap's `possible_values(..)` function.
+            _ => unreachable!(),
+        };
+    }
+
     // Log configuration
     info!(log, "";
           "data_dir" => &config.data_dir.to_str(),
@@ -81,11 +102,8 @@ fn main() {
         Arc::new(ValidatorServiceClient::new(ch))
     };
 
-    // Ethereum
-    //
-    // TODO: Permit loading a custom spec from file.
-    // https://github.com/sigp/lighthouse/issues/160
-    let spec = Arc::new(ChainSpec::foundation());
+    // Spec
+    let spec = Arc::new(config.spec.clone());
 
     // Clock for determining the present slot.
     // TODO: this shouldn't be a static time, instead it should be pulled from the beacon node.