diff --git a/Cargo.toml b/Cargo.toml
index c05e22286..37ec4ad0f 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -10,6 +10,7 @@ members = [
 	"eth2/utils/bls",
 	"eth2/utils/boolean-bitfield",
 	"eth2/utils/cached_tree_hash",
+	"eth2/utils/fixed_len_vec",
 	"eth2/utils/hashing",
 	"eth2/utils/honey-badger-split",
 	"eth2/utils/merkle_proof",
diff --git a/eth2/types/Cargo.toml b/eth2/types/Cargo.toml
index a2bd08d61..160697edd 100644
--- a/eth2/types/Cargo.toml
+++ b/eth2/types/Cargo.toml
@@ -11,6 +11,7 @@ cached_tree_hash = { path = "../utils/cached_tree_hash" }
 dirs = "1.0"
 derivative = "1.0"
 ethereum-types = "0.5"
+fixed_len_vec = { path = "../utils/fixed_len_vec" }
 hashing = { path = "../utils/hashing" }
 hex = "0.3"
 honey-badger-split =  { path = "../utils/honey-badger-split" }
@@ -29,7 +30,6 @@ swap_or_not_shuffle = { path = "../utils/swap_or_not_shuffle" }
 test_random_derive = { path = "../utils/test_random_derive" }
 tree_hash = { path = "../utils/tree_hash" }
 tree_hash_derive = { path = "../utils/tree_hash_derive" }
-typenum = "1.10"
 libp2p =  { git = "https://github.com/SigP/rust-libp2p", rev = "b3c32d9a821ae6cc89079499cc6e8a6bab0bffc3" }
 
 [dev-dependencies]
diff --git a/eth2/types/src/attestation.rs b/eth2/types/src/attestation.rs
index d1511763d..d6d5d3a22 100644
--- a/eth2/types/src/attestation.rs
+++ b/eth2/types/src/attestation.rs
@@ -1,6 +1,6 @@
 use super::{AggregateSignature, AttestationData, Bitfield};
 use crate::test_utils::TestRandom;
-use rand::RngCore;
+
 use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode};
 use test_random_derive::TestRandom;
diff --git a/eth2/types/src/attestation_data.rs b/eth2/types/src/attestation_data.rs
index c963d6001..60e539ab8 100644
--- a/eth2/types/src/attestation_data.rs
+++ b/eth2/types/src/attestation_data.rs
@@ -1,6 +1,6 @@
 use crate::test_utils::TestRandom;
 use crate::{Crosslink, Epoch, Hash256, Slot};
-use rand::RngCore;
+
 use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode};
 use test_random_derive::TestRandom;
diff --git a/eth2/types/src/attestation_data_and_custody_bit.rs b/eth2/types/src/attestation_data_and_custody_bit.rs
index 85a5875ab..f1437cb54 100644
--- a/eth2/types/src/attestation_data_and_custody_bit.rs
+++ b/eth2/types/src/attestation_data_and_custody_bit.rs
@@ -1,5 +1,6 @@
 use super::AttestationData;
 use crate::test_utils::TestRandom;
+
 use rand::RngCore;
 use serde_derive::Serialize;
 use ssz_derive::{Decode, Encode};
@@ -14,8 +15,8 @@ pub struct AttestationDataAndCustodyBit {
     pub custody_bit: bool,
 }
 
-impl<T: RngCore> TestRandom<T> for AttestationDataAndCustodyBit {
-    fn random_for_test(rng: &mut T) -> Self {
+impl TestRandom for AttestationDataAndCustodyBit {
+    fn random_for_test(rng: &mut impl RngCore) -> Self {
         Self {
             data: <_>::random_for_test(rng),
             custody_bit: <_>::random_for_test(rng),
diff --git a/eth2/types/src/attester_slashing.rs b/eth2/types/src/attester_slashing.rs
index d4848b01c..d7b5d5942 100644
--- a/eth2/types/src/attester_slashing.rs
+++ b/eth2/types/src/attester_slashing.rs
@@ -1,5 +1,5 @@
 use crate::{test_utils::TestRandom, SlashableAttestation};
-use rand::RngCore;
+
 use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode};
 use test_random_derive::TestRandom;
diff --git a/eth2/types/src/beacon_block.rs b/eth2/types/src/beacon_block.rs
index d198d16fc..33d73db16 100644
--- a/eth2/types/src/beacon_block.rs
+++ b/eth2/types/src/beacon_block.rs
@@ -1,7 +1,7 @@
 use crate::test_utils::TestRandom;
 use crate::*;
 use bls::Signature;
-use rand::RngCore;
+
 use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode};
 use test_random_derive::TestRandom;
diff --git a/eth2/types/src/beacon_block_body.rs b/eth2/types/src/beacon_block_body.rs
index 15ba00d6b..867db78c4 100644
--- a/eth2/types/src/beacon_block_body.rs
+++ b/eth2/types/src/beacon_block_body.rs
@@ -1,6 +1,6 @@
 use crate::test_utils::TestRandom;
 use crate::*;
-use rand::RngCore;
+
 use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode};
 use test_random_derive::TestRandom;
diff --git a/eth2/types/src/beacon_block_header.rs b/eth2/types/src/beacon_block_header.rs
index 5b35da1b6..601346db5 100644
--- a/eth2/types/src/beacon_block_header.rs
+++ b/eth2/types/src/beacon_block_header.rs
@@ -1,7 +1,7 @@
 use crate::test_utils::TestRandom;
 use crate::*;
 use bls::Signature;
-use rand::RngCore;
+
 use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode};
 use test_random_derive::TestRandom;
diff --git a/eth2/types/src/beacon_state.rs b/eth2/types/src/beacon_state.rs
index 29c9d9b31..46c4ae87d 100644
--- a/eth2/types/src/beacon_state.rs
+++ b/eth2/types/src/beacon_state.rs
@@ -4,7 +4,8 @@ use crate::*;
 use cached_tree_hash::{Error as TreeHashCacheError, TreeHashCache};
 use int_to_bytes::int_to_bytes32;
 use pubkey_cache::PubkeyCache;
-use rand::RngCore;
+
+use fixed_len_vec::FixedLenVec;
 use serde_derive::{Deserialize, Serialize};
 use ssz::{hash, ssz_encode};
 use ssz_derive::{Decode, Encode};
@@ -12,8 +13,10 @@ use test_random_derive::TestRandom;
 use tree_hash::TreeHash;
 use tree_hash_derive::{CachedTreeHash, TreeHash};
 
+pub use beacon_state_types::{BeaconStateTypes, FoundationBeaconState};
+
+mod beacon_state_types;
 mod epoch_cache;
-mod fixed_params;
 mod pubkey_cache;
 mod tests;
 
@@ -62,7 +65,10 @@ pub enum Error {
     TreeHash,
     CachedTreeHash,
 )]
-pub struct BeaconState {
+pub struct BeaconState<T>
+where
+    T: BeaconStateTypes,
+{
     // Misc
     pub slot: Slot,
     pub genesis_time: u64,
@@ -74,7 +80,7 @@ pub struct BeaconState {
     pub validator_registry_update_epoch: Epoch,
 
     // Randomness and committees
-    pub latest_randao_mixes: TreeHashVector<Hash256>,
+    pub latest_randao_mixes: FixedLenVec<Hash256, T::NumLatestRandaoMixes>,
     pub previous_shuffling_start_shard: u64,
     pub current_shuffling_start_shard: u64,
     pub previous_shuffling_epoch: Epoch,
@@ -134,14 +140,18 @@ pub struct BeaconState {
     pub tree_hash_cache: TreeHashCache,
 }
 
-impl BeaconState {
+impl<T: BeaconStateTypes> BeaconState<T> {
     /// Produce the first state of the Beacon Chain.
     ///
     /// This does not fully build a genesis beacon state, it omits processing of initial validator
     /// deposits. To obtain a full genesis beacon state, use the `BeaconStateBuilder`.
     ///
     /// Spec v0.5.1
-    pub fn genesis(genesis_time: u64, latest_eth1_data: Eth1Data, spec: &ChainSpec) -> BeaconState {
+    pub fn genesis(
+        genesis_time: u64,
+        latest_eth1_data: Eth1Data,
+        spec: &ChainSpec,
+    ) -> BeaconState<T> {
         let initial_crosslink = Crosslink {
             epoch: spec.genesis_epoch,
             crosslink_data_root: spec.zero_hash,
@@ -425,7 +435,8 @@ impl BeaconState {
             & (epoch <= current_epoch)
         {
             let i = epoch.as_usize() % spec.latest_randao_mixes_length;
-            if i < self.latest_randao_mixes.len() {
+
+            if i < (&self.latest_randao_mixes[..]).len() {
                 Ok(i)
             } else {
                 Err(Error::InsufficientRandaoMixes)
diff --git a/eth2/types/src/beacon_state/beacon_state_types.rs b/eth2/types/src/beacon_state/beacon_state_types.rs
new file mode 100644
index 000000000..63c038575
--- /dev/null
+++ b/eth2/types/src/beacon_state/beacon_state_types.rs
@@ -0,0 +1,15 @@
+use crate::*;
+use fixed_len_vec::typenum::{Unsigned, U8192};
+
+pub trait BeaconStateTypes {
+    type NumLatestRandaoMixes: Unsigned + Clone + Sync + Send;
+}
+
+#[derive(Clone, PartialEq, Debug)]
+pub struct FoundationStateParams;
+
+impl BeaconStateTypes for FoundationStateParams {
+    type NumLatestRandaoMixes = U8192;
+}
+
+pub type FoundationBeaconState = BeaconState<FoundationStateParams>;
diff --git a/eth2/types/src/beacon_state/epoch_cache.rs b/eth2/types/src/beacon_state/epoch_cache.rs
index 1a63e9eb9..31becc5d3 100644
--- a/eth2/types/src/beacon_state/epoch_cache.rs
+++ b/eth2/types/src/beacon_state/epoch_cache.rs
@@ -28,8 +28,8 @@ pub struct EpochCache {
 
 impl EpochCache {
     /// Return a new, fully initialized cache.
-    pub fn initialized(
-        state: &BeaconState,
+    pub fn initialized<T: BeaconStateTypes>(
+        state: &BeaconState<T>,
         relative_epoch: RelativeEpoch,
         spec: &ChainSpec,
     ) -> Result<EpochCache, Error> {
@@ -200,8 +200,8 @@ pub struct EpochCrosslinkCommitteesBuilder {
 
 impl EpochCrosslinkCommitteesBuilder {
     /// Instantiates a builder that will build for the `state`'s previous epoch.
-    pub fn for_previous_epoch(
-        state: &BeaconState,
+    pub fn for_previous_epoch<T: BeaconStateTypes>(
+        state: &BeaconState<T>,
         active_validator_indices: Vec<usize>,
         spec: &ChainSpec,
     ) -> Self {
@@ -215,8 +215,8 @@ impl EpochCrosslinkCommitteesBuilder {
     }
 
     /// Instantiates a builder that will build for the `state`'s next epoch.
-    pub fn for_current_epoch(
-        state: &BeaconState,
+    pub fn for_current_epoch<T: BeaconStateTypes>(
+        state: &BeaconState<T>,
         active_validator_indices: Vec<usize>,
         spec: &ChainSpec,
     ) -> Self {
@@ -233,8 +233,8 @@ impl EpochCrosslinkCommitteesBuilder {
     ///
     /// Note: there are two possible epoch builds for the next epoch, one where there is a registry
     /// change and one where there is not.
-    pub fn for_next_epoch(
-        state: &BeaconState,
+    pub fn for_next_epoch<T: BeaconStateTypes>(
+        state: &BeaconState<T>,
         active_validator_indices: Vec<usize>,
         registry_change: bool,
         spec: &ChainSpec,
diff --git a/eth2/types/src/beacon_state/epoch_cache/tests.rs b/eth2/types/src/beacon_state/epoch_cache/tests.rs
index 5b1e53338..e196560a3 100644
--- a/eth2/types/src/beacon_state/epoch_cache/tests.rs
+++ b/eth2/types/src/beacon_state/epoch_cache/tests.rs
@@ -4,8 +4,8 @@ use super::*;
 use crate::test_utils::*;
 use swap_or_not_shuffle::shuffle_list;
 
-fn do_sane_cache_test(
-    state: BeaconState,
+fn do_sane_cache_test<T: BeaconStateTypes>(
+    state: BeaconState<T>,
     epoch: Epoch,
     relative_epoch: RelativeEpoch,
     validator_count: usize,
@@ -64,7 +64,10 @@ fn do_sane_cache_test(
     }
 }
 
-fn setup_sane_cache_test(validator_count: usize, spec: &ChainSpec) -> BeaconState {
+fn setup_sane_cache_test<T: BeaconStateTypes>(
+    validator_count: usize,
+    spec: &ChainSpec,
+) -> BeaconState<T> {
     let mut builder =
         TestingBeaconStateBuilder::from_default_keypairs_file_if_exists(validator_count, spec);
 
@@ -101,7 +104,7 @@ fn builds_sane_current_epoch_cache() {
     let mut spec = ChainSpec::few_validators();
     spec.shard_count = 4;
     let validator_count = (spec.shard_count * spec.target_committee_size) + 1;
-    let state = setup_sane_cache_test(validator_count as usize, &spec);
+    let state: FoundationBeaconState = setup_sane_cache_test(validator_count as usize, &spec);
     do_sane_cache_test(
         state.clone(),
         state.current_epoch(&spec),
@@ -118,7 +121,7 @@ fn builds_sane_previous_epoch_cache() {
     let mut spec = ChainSpec::few_validators();
     spec.shard_count = 2;
     let validator_count = (spec.shard_count * spec.target_committee_size) + 1;
-    let state = setup_sane_cache_test(validator_count as usize, &spec);
+    let state: FoundationBeaconState = setup_sane_cache_test(validator_count as usize, &spec);
     do_sane_cache_test(
         state.clone(),
         state.previous_epoch(&spec),
@@ -135,7 +138,7 @@ fn builds_sane_next_without_update_epoch_cache() {
     let mut spec = ChainSpec::few_validators();
     spec.shard_count = 2;
     let validator_count = (spec.shard_count * spec.target_committee_size) + 1;
-    let mut state = setup_sane_cache_test(validator_count as usize, &spec);
+    let mut state: FoundationBeaconState = setup_sane_cache_test(validator_count as usize, &spec);
     state.validator_registry_update_epoch = state.slot.epoch(spec.slots_per_epoch);
     do_sane_cache_test(
         state.clone(),
diff --git a/eth2/types/src/beacon_state/tests.rs b/eth2/types/src/beacon_state/tests.rs
index d5862559a..759061498 100644
--- a/eth2/types/src/beacon_state/tests.rs
+++ b/eth2/types/src/beacon_state/tests.rs
@@ -2,16 +2,16 @@
 use super::*;
 use crate::test_utils::*;
 
-ssz_tests!(BeaconState);
-cached_tree_hash_tests!(BeaconState);
+ssz_tests!(FoundationBeaconState);
+cached_tree_hash_tests!(FoundationBeaconState);
 
 /// Test that
 ///
 /// 1. Using the cache before it's built fails.
 /// 2. Using the cache after it's build passes.
 /// 3. Using the cache after it's dropped fails.
-fn test_cache_initialization<'a>(
-    state: &'a mut BeaconState,
+fn test_cache_initialization<'a, T: BeaconStateTypes>(
+    state: &'a mut BeaconState<T>,
     relative_epoch: RelativeEpoch,
     spec: &ChainSpec,
 ) {
@@ -46,7 +46,7 @@ fn test_cache_initialization<'a>(
 #[test]
 fn cache_initialization() {
     let spec = ChainSpec::few_validators();
-    let (mut state, _keypairs) =
+    let (mut state, _keypairs): (FoundationBeaconState, Vec<Keypair>) =
         TestingBeaconStateBuilder::from_default_keypairs_file_if_exists(16, &spec).build();
 
     state.slot = (spec.genesis_epoch + 1).start_slot(spec.slots_per_epoch);
@@ -64,7 +64,7 @@ fn tree_hash_cache() {
 
     let mut rng = XorShiftRng::from_seed([42; 16]);
 
-    let mut state = BeaconState::random_for_test(&mut rng);
+    let mut state: FoundationBeaconState = BeaconState::random_for_test(&mut rng);
 
     let root = state.update_tree_hash_cache().unwrap();
 
diff --git a/eth2/types/src/crosslink.rs b/eth2/types/src/crosslink.rs
index 448f5ea30..cb3ce5615 100644
--- a/eth2/types/src/crosslink.rs
+++ b/eth2/types/src/crosslink.rs
@@ -1,6 +1,6 @@
 use crate::test_utils::TestRandom;
 use crate::{Epoch, Hash256};
-use rand::RngCore;
+
 use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode};
 use test_random_derive::TestRandom;
diff --git a/eth2/types/src/deposit.rs b/eth2/types/src/deposit.rs
index e8d2f5a4b..1f5d90fab 100644
--- a/eth2/types/src/deposit.rs
+++ b/eth2/types/src/deposit.rs
@@ -1,6 +1,6 @@
 use super::{DepositData, Hash256, TreeHashVector};
 use crate::test_utils::TestRandom;
-use rand::RngCore;
+
 use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode};
 use test_random_derive::TestRandom;
diff --git a/eth2/types/src/deposit_data.rs b/eth2/types/src/deposit_data.rs
index 38c44d1a7..bee1d503e 100644
--- a/eth2/types/src/deposit_data.rs
+++ b/eth2/types/src/deposit_data.rs
@@ -1,6 +1,6 @@
 use super::DepositInput;
 use crate::test_utils::TestRandom;
-use rand::RngCore;
+
 use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode};
 use test_random_derive::TestRandom;
diff --git a/eth2/types/src/deposit_input.rs b/eth2/types/src/deposit_input.rs
index af1049a20..f44c75f5a 100644
--- a/eth2/types/src/deposit_input.rs
+++ b/eth2/types/src/deposit_input.rs
@@ -1,7 +1,7 @@
 use crate::test_utils::TestRandom;
 use crate::*;
 use bls::{PublicKey, Signature};
-use rand::RngCore;
+
 use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode};
 use test_random_derive::TestRandom;
diff --git a/eth2/types/src/eth1_data.rs b/eth2/types/src/eth1_data.rs
index 3c0c3af02..aaf5bca54 100644
--- a/eth2/types/src/eth1_data.rs
+++ b/eth2/types/src/eth1_data.rs
@@ -1,6 +1,6 @@
 use super::Hash256;
 use crate::test_utils::TestRandom;
-use rand::RngCore;
+
 use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode};
 use test_random_derive::TestRandom;
diff --git a/eth2/types/src/eth1_data_vote.rs b/eth2/types/src/eth1_data_vote.rs
index 00818ebf4..27cb0be78 100644
--- a/eth2/types/src/eth1_data_vote.rs
+++ b/eth2/types/src/eth1_data_vote.rs
@@ -1,6 +1,6 @@
 use super::Eth1Data;
 use crate::test_utils::TestRandom;
-use rand::RngCore;
+
 use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode};
 use test_random_derive::TestRandom;
diff --git a/eth2/types/src/fork.rs b/eth2/types/src/fork.rs
index 83d4f5dc6..3e7254dd3 100644
--- a/eth2/types/src/fork.rs
+++ b/eth2/types/src/fork.rs
@@ -3,7 +3,7 @@ use crate::{
     ChainSpec, Epoch,
 };
 use int_to_bytes::int_to_bytes4;
-use rand::RngCore;
+
 use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode};
 use test_random_derive::TestRandom;
diff --git a/eth2/types/src/historical_batch.rs b/eth2/types/src/historical_batch.rs
index 13e57131a..3f8baabbc 100644
--- a/eth2/types/src/historical_batch.rs
+++ b/eth2/types/src/historical_batch.rs
@@ -1,6 +1,6 @@
 use crate::test_utils::TestRandom;
 use crate::{Hash256, TreeHashVector};
-use rand::RngCore;
+
 use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode};
 use test_random_derive::TestRandom;
diff --git a/eth2/types/src/lib.rs b/eth2/types/src/lib.rs
index 070ed6745..efe7f2327 100644
--- a/eth2/types/src/lib.rs
+++ b/eth2/types/src/lib.rs
@@ -36,6 +36,7 @@ pub mod slot_epoch;
 pub mod slot_height;
 pub mod validator;
 
+use beacon_state::FoundationBeaconState;
 use ethereum_types::{H160, H256, U256};
 use std::collections::HashMap;
 
@@ -47,7 +48,7 @@ pub use crate::attester_slashing::AttesterSlashing;
 pub use crate::beacon_block::BeaconBlock;
 pub use crate::beacon_block_body::BeaconBlockBody;
 pub use crate::beacon_block_header::BeaconBlockHeader;
-pub use crate::beacon_state::{BeaconState, Error as BeaconStateError};
+pub use crate::beacon_state::{BeaconState, BeaconStateTypes, Error as BeaconStateError};
 pub use crate::chain_spec::{ChainSpec, Domain};
 pub use crate::crosslink::Crosslink;
 pub use crate::crosslink_committee::CrosslinkCommittee;
@@ -87,6 +88,7 @@ pub type AttesterMap = HashMap<(u64, u64), Vec<usize>>;
 pub type ProposerMap = HashMap<u64, usize>;
 
 pub use bls::{AggregatePublicKey, AggregateSignature, Keypair, PublicKey, SecretKey, Signature};
+pub use fixed_len_vec::FixedLenVec;
 pub use libp2p::floodsub::{Topic, TopicBuilder, TopicHash};
 pub use libp2p::multiaddr;
 pub use libp2p::Multiaddr;
diff --git a/eth2/types/src/pending_attestation.rs b/eth2/types/src/pending_attestation.rs
index b71351f9a..b158f1274 100644
--- a/eth2/types/src/pending_attestation.rs
+++ b/eth2/types/src/pending_attestation.rs
@@ -1,6 +1,6 @@
 use crate::test_utils::TestRandom;
 use crate::{Attestation, AttestationData, Bitfield, Slot};
-use rand::RngCore;
+
 use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode};
 use test_random_derive::TestRandom;
diff --git a/eth2/types/src/proposer_slashing.rs b/eth2/types/src/proposer_slashing.rs
index bf26ae508..0c419dd56 100644
--- a/eth2/types/src/proposer_slashing.rs
+++ b/eth2/types/src/proposer_slashing.rs
@@ -1,6 +1,6 @@
 use super::BeaconBlockHeader;
 use crate::test_utils::TestRandom;
-use rand::RngCore;
+
 use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode};
 use test_random_derive::TestRandom;
diff --git a/eth2/types/src/slashable_attestation.rs b/eth2/types/src/slashable_attestation.rs
index fb838e0c4..19d9a87b6 100644
--- a/eth2/types/src/slashable_attestation.rs
+++ b/eth2/types/src/slashable_attestation.rs
@@ -1,5 +1,5 @@
 use crate::{test_utils::TestRandom, AggregateSignature, AttestationData, Bitfield, ChainSpec};
-use rand::RngCore;
+
 use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode};
 use test_random_derive::TestRandom;
diff --git a/eth2/types/src/slot_epoch.rs b/eth2/types/src/slot_epoch.rs
index 6c6a92ecb..7f1f704ca 100644
--- a/eth2/types/src/slot_epoch.rs
+++ b/eth2/types/src/slot_epoch.rs
@@ -1,15 +1,16 @@
+//! The `Slot` and `Epoch` types are defined as newtypes over u64 to enforce type-safety between
+//! the two types.
+//!
+//! `Slot` and `Epoch` have implementations which permit conversion, comparison and math operations
+//! between each and `u64`, however specifically not between each other.
+//!
+//! All math operations on `Slot` and `Epoch` are saturating, they never wrap.
+//!
+//! It would be easy to define `PartialOrd` and other traits generically across all types which
+//! implement `Into<u64>`, however this would allow operations between `Slots` and `Epochs` which
+//! may lead to programming errors which are not detected by the compiler.
+
 use crate::slot_height::SlotHeight;
-/// The `Slot` and `Epoch` types are defined as newtypes over u64 to enforce type-safety between
-/// the two types.
-///
-/// `Slot` and `Epoch` have implementations which permit conversion, comparison and math operations
-/// between each and `u64`, however specifically not between each other.
-///
-/// All math operations on `Slot` and `Epoch` are saturating, they never wrap.
-///
-/// It would be easy to define `PartialOrd` and other traits generically across all types which
-/// implement `Into<u64>`, however this would allow operations between `Slots` and `Epochs` which
-/// may lead to programming errors which are not detected by the compiler.
 use crate::test_utils::TestRandom;
 use rand::RngCore;
 use serde_derive::{Deserialize, Serialize};
diff --git a/eth2/types/src/slot_epoch_macros.rs b/eth2/types/src/slot_epoch_macros.rs
index 4a48bba9f..076efbb77 100644
--- a/eth2/types/src/slot_epoch_macros.rs
+++ b/eth2/types/src/slot_epoch_macros.rs
@@ -244,8 +244,8 @@ macro_rules! impl_ssz {
             }
         }
 
-        impl<T: RngCore> TestRandom<T> for $type {
-            fn random_for_test(rng: &mut T) -> Self {
+        impl TestRandom for $type {
+            fn random_for_test(rng: &mut impl RngCore) -> Self {
                 $type::from(u64::random_for_test(rng))
             }
         }
diff --git a/eth2/types/src/slot_height.rs b/eth2/types/src/slot_height.rs
index f7a34cbba..b0d5d65d9 100644
--- a/eth2/types/src/slot_height.rs
+++ b/eth2/types/src/slot_height.rs
@@ -1,5 +1,6 @@
 use crate::slot_epoch::{Epoch, Slot};
 use crate::test_utils::TestRandom;
+
 use rand::RngCore;
 use serde_derive::Serialize;
 use ssz::{ssz_encode, Decodable, DecodeError, Encodable, SszStream};
diff --git a/eth2/types/src/test_utils/test_random.rs b/eth2/types/src/test_utils/test_random.rs
index 2d4269b08..ceb785424 100644
--- a/eth2/types/src/test_utils/test_random.rs
+++ b/eth2/types/src/test_utils/test_random.rs
@@ -1,3 +1,5 @@
+use crate::*;
+use fixed_len_vec::typenum::Unsigned;
 use rand::RngCore;
 
 mod address;
@@ -8,42 +10,39 @@ mod public_key;
 mod secret_key;
 mod signature;
 
-pub trait TestRandom<T>
-where
-    T: RngCore,
-{
-    fn random_for_test(rng: &mut T) -> Self;
+pub trait TestRandom {
+    fn random_for_test(rng: &mut impl RngCore) -> Self;
 }
 
-impl<T: RngCore> TestRandom<T> for bool {
-    fn random_for_test(rng: &mut T) -> Self {
+impl TestRandom for bool {
+    fn random_for_test(rng: &mut impl RngCore) -> Self {
         (rng.next_u32() % 2) == 1
     }
 }
 
-impl<T: RngCore> TestRandom<T> for u64 {
-    fn random_for_test(rng: &mut T) -> Self {
+impl TestRandom for u64 {
+    fn random_for_test(rng: &mut impl RngCore) -> Self {
         rng.next_u64()
     }
 }
 
-impl<T: RngCore> TestRandom<T> for u32 {
-    fn random_for_test(rng: &mut T) -> Self {
+impl TestRandom for u32 {
+    fn random_for_test(rng: &mut impl RngCore) -> Self {
         rng.next_u32()
     }
 }
 
-impl<T: RngCore> TestRandom<T> for usize {
-    fn random_for_test(rng: &mut T) -> Self {
+impl TestRandom for usize {
+    fn random_for_test(rng: &mut impl RngCore) -> Self {
         rng.next_u32() as usize
     }
 }
 
-impl<T: RngCore, U> TestRandom<T> for Vec<U>
+impl<U> TestRandom for Vec<U>
 where
-    U: TestRandom<T>,
+    U: TestRandom,
 {
-    fn random_for_test(rng: &mut T) -> Self {
+    fn random_for_test(rng: &mut impl RngCore) -> Self {
         let mut output = vec![];
 
         for _ in 0..(usize::random_for_test(rng) % 4) {
@@ -54,10 +53,25 @@ where
     }
 }
 
+impl<T, N: Unsigned> TestRandom for FixedLenVec<T, N>
+where
+    T: TestRandom + Default,
+{
+    fn random_for_test(rng: &mut impl RngCore) -> Self {
+        let mut output = vec![];
+
+        for _ in 0..(usize::random_for_test(rng) % std::cmp::min(4, N::to_usize())) {
+            output.push(<T>::random_for_test(rng));
+        }
+
+        output.into()
+    }
+}
+
 macro_rules! impl_test_random_for_u8_array {
     ($len: expr) => {
-        impl<T: RngCore> TestRandom<T> for [u8; $len] {
-            fn random_for_test(rng: &mut T) -> Self {
+        impl TestRandom for [u8; $len] {
+            fn random_for_test(rng: &mut impl RngCore) -> Self {
                 let mut bytes = [0; $len];
                 rng.fill_bytes(&mut bytes);
                 bytes
diff --git a/eth2/types/src/test_utils/test_random/address.rs b/eth2/types/src/test_utils/test_random/address.rs
index 13de2dec9..3aaad307e 100644
--- a/eth2/types/src/test_utils/test_random/address.rs
+++ b/eth2/types/src/test_utils/test_random/address.rs
@@ -1,9 +1,8 @@
-use super::TestRandom;
+use super::*;
 use crate::Address;
-use rand::RngCore;
 
-impl<T: RngCore> TestRandom<T> for Address {
-    fn random_for_test(rng: &mut T) -> Self {
+impl TestRandom for Address {
+    fn random_for_test(rng: &mut impl RngCore) -> Self {
         let mut key_bytes = vec![0; 20];
         rng.fill_bytes(&mut key_bytes);
         Address::from_slice(&key_bytes[..])
diff --git a/eth2/types/src/test_utils/test_random/aggregate_signature.rs b/eth2/types/src/test_utils/test_random/aggregate_signature.rs
index 6a15f7366..a346d2d88 100644
--- a/eth2/types/src/test_utils/test_random/aggregate_signature.rs
+++ b/eth2/types/src/test_utils/test_random/aggregate_signature.rs
@@ -1,9 +1,8 @@
-use super::TestRandom;
+use super::*;
 use bls::{AggregateSignature, Signature};
-use rand::RngCore;
 
-impl<T: RngCore> TestRandom<T> for AggregateSignature {
-    fn random_for_test(rng: &mut T) -> Self {
+impl TestRandom for AggregateSignature {
+    fn random_for_test(rng: &mut impl RngCore) -> Self {
         let signature = Signature::random_for_test(rng);
         let mut aggregate_signature = AggregateSignature::new();
         aggregate_signature.add(&signature);
diff --git a/eth2/types/src/test_utils/test_random/bitfield.rs b/eth2/types/src/test_utils/test_random/bitfield.rs
index 9748458f1..9a4d21840 100644
--- a/eth2/types/src/test_utils/test_random/bitfield.rs
+++ b/eth2/types/src/test_utils/test_random/bitfield.rs
@@ -1,9 +1,8 @@
-use super::TestRandom;
+use super::*;
 use crate::Bitfield;
-use rand::RngCore;
 
-impl<T: RngCore> TestRandom<T> for Bitfield {
-    fn random_for_test(rng: &mut T) -> Self {
+impl TestRandom for Bitfield {
+    fn random_for_test(rng: &mut impl RngCore) -> Self {
         let mut raw_bytes = vec![0; 32];
         rng.fill_bytes(&mut raw_bytes);
         Bitfield::from_bytes(&raw_bytes)
diff --git a/eth2/types/src/test_utils/test_random/hash256.rs b/eth2/types/src/test_utils/test_random/hash256.rs
index a227679da..8733f7de2 100644
--- a/eth2/types/src/test_utils/test_random/hash256.rs
+++ b/eth2/types/src/test_utils/test_random/hash256.rs
@@ -1,9 +1,8 @@
-use super::TestRandom;
+use super::*;
 use crate::Hash256;
-use rand::RngCore;
 
-impl<T: RngCore> TestRandom<T> for Hash256 {
-    fn random_for_test(rng: &mut T) -> Self {
+impl TestRandom for Hash256 {
+    fn random_for_test(rng: &mut impl RngCore) -> Self {
         let mut key_bytes = vec![0; 32];
         rng.fill_bytes(&mut key_bytes);
         Hash256::from_slice(&key_bytes[..])
diff --git a/eth2/types/src/test_utils/test_random/public_key.rs b/eth2/types/src/test_utils/test_random/public_key.rs
index bfccd3e53..d643eaf0b 100644
--- a/eth2/types/src/test_utils/test_random/public_key.rs
+++ b/eth2/types/src/test_utils/test_random/public_key.rs
@@ -1,9 +1,8 @@
-use super::TestRandom;
+use super::*;
 use bls::{PublicKey, SecretKey};
-use rand::RngCore;
 
-impl<T: RngCore> TestRandom<T> for PublicKey {
-    fn random_for_test(rng: &mut T) -> Self {
+impl TestRandom for PublicKey {
+    fn random_for_test(rng: &mut impl RngCore) -> Self {
         let secret_key = SecretKey::random_for_test(rng);
         PublicKey::from_secret_key(&secret_key)
     }
diff --git a/eth2/types/src/test_utils/test_random/secret_key.rs b/eth2/types/src/test_utils/test_random/secret_key.rs
index 17481c3de..a833a4488 100644
--- a/eth2/types/src/test_utils/test_random/secret_key.rs
+++ b/eth2/types/src/test_utils/test_random/secret_key.rs
@@ -1,9 +1,8 @@
-use super::TestRandom;
+use super::*;
 use bls::SecretKey;
-use rand::RngCore;
 
-impl<T: RngCore> TestRandom<T> for SecretKey {
-    fn random_for_test(rng: &mut T) -> Self {
+impl TestRandom for SecretKey {
+    fn random_for_test(rng: &mut impl RngCore) -> Self {
         let mut key_bytes = vec![0; 48];
         rng.fill_bytes(&mut key_bytes);
         /*
diff --git a/eth2/types/src/test_utils/test_random/signature.rs b/eth2/types/src/test_utils/test_random/signature.rs
index d9995835a..ef5d9a17f 100644
--- a/eth2/types/src/test_utils/test_random/signature.rs
+++ b/eth2/types/src/test_utils/test_random/signature.rs
@@ -1,9 +1,8 @@
-use super::TestRandom;
+use super::*;
 use bls::{SecretKey, Signature};
-use rand::RngCore;
 
-impl<T: RngCore> TestRandom<T> for Signature {
-    fn random_for_test(rng: &mut T) -> Self {
+impl TestRandom for Signature {
+    fn random_for_test(rng: &mut impl RngCore) -> Self {
         let secret_key = SecretKey::random_for_test(rng);
         let mut message = vec![0; 32];
         rng.fill_bytes(&mut message);
diff --git a/eth2/types/src/test_utils/testing_attestation_builder.rs b/eth2/types/src/test_utils/testing_attestation_builder.rs
index 162facc8e..2d9b5ac6f 100644
--- a/eth2/types/src/test_utils/testing_attestation_builder.rs
+++ b/eth2/types/src/test_utils/testing_attestation_builder.rs
@@ -12,8 +12,8 @@ pub struct TestingAttestationBuilder {
 
 impl TestingAttestationBuilder {
     /// Create a new attestation builder.
-    pub fn new(
-        state: &BeaconState,
+    pub fn new<T: BeaconStateTypes>(
+        state: &BeaconState<T>,
         committee: &[usize],
         slot: Slot,
         shard: u64,
diff --git a/eth2/types/src/test_utils/testing_attestation_data_builder.rs b/eth2/types/src/test_utils/testing_attestation_data_builder.rs
index a270e3859..4cb83a6b8 100644
--- a/eth2/types/src/test_utils/testing_attestation_data_builder.rs
+++ b/eth2/types/src/test_utils/testing_attestation_data_builder.rs
@@ -10,7 +10,12 @@ pub struct TestingAttestationDataBuilder {
 impl TestingAttestationDataBuilder {
     /// Configures a new `AttestationData` which attests to all of the same parameters as the
     /// state.
-    pub fn new(state: &BeaconState, shard: u64, slot: Slot, spec: &ChainSpec) -> Self {
+    pub fn new<T: BeaconStateTypes>(
+        state: &BeaconState<T>,
+        shard: u64,
+        slot: Slot,
+        spec: &ChainSpec,
+    ) -> Self {
         let current_epoch = state.current_epoch(spec);
         let previous_epoch = state.previous_epoch(spec);
 
diff --git a/eth2/types/src/test_utils/testing_beacon_block_builder.rs b/eth2/types/src/test_utils/testing_beacon_block_builder.rs
index 549c00ac0..a2ea65949 100644
--- a/eth2/types/src/test_utils/testing_beacon_block_builder.rs
+++ b/eth2/types/src/test_utils/testing_beacon_block_builder.rs
@@ -82,9 +82,9 @@ impl TestingBeaconBlockBuilder {
     ///
     /// Note: the signed messages of the split committees will be identical -- it would be possible
     /// to aggregate these split attestations.
-    pub fn insert_attestations(
+    pub fn insert_attestations<T: BeaconStateTypes>(
         &mut self,
-        state: &BeaconState,
+        state: &BeaconState<T>,
         secret_keys: &[&SecretKey],
         num_attestations: usize,
         spec: &ChainSpec,
@@ -171,11 +171,11 @@ impl TestingBeaconBlockBuilder {
     }
 
     /// Insert a `Valid` deposit into the state.
-    pub fn insert_deposit(
+    pub fn insert_deposit<T: BeaconStateTypes>(
         &mut self,
         amount: u64,
         index: u64,
-        state: &BeaconState,
+        state: &BeaconState<T>,
         spec: &ChainSpec,
     ) {
         let keypair = Keypair::random();
@@ -193,9 +193,9 @@ impl TestingBeaconBlockBuilder {
     }
 
     /// Insert a `Valid` exit into the state.
-    pub fn insert_exit(
+    pub fn insert_exit<T: BeaconStateTypes>(
         &mut self,
-        state: &BeaconState,
+        state: &BeaconState<T>,
         validator_index: u64,
         secret_key: &SecretKey,
         spec: &ChainSpec,
@@ -214,9 +214,9 @@ impl TestingBeaconBlockBuilder {
     ///
     /// Note: this will set the validator to be withdrawable by directly modifying the state
     /// validator registry. This _may_ cause problems historic hashes, etc.
-    pub fn insert_transfer(
+    pub fn insert_transfer<T: BeaconStateTypes>(
         &mut self,
-        state: &BeaconState,
+        state: &BeaconState<T>,
         from: u64,
         to: u64,
         amount: u64,
diff --git a/eth2/types/src/test_utils/testing_beacon_state_builder.rs b/eth2/types/src/test_utils/testing_beacon_state_builder.rs
index 9bdd9e149..67f23a44c 100644
--- a/eth2/types/src/test_utils/testing_beacon_state_builder.rs
+++ b/eth2/types/src/test_utils/testing_beacon_state_builder.rs
@@ -25,12 +25,12 @@ pub fn keypairs_path() -> PathBuf {
 ///
 /// This struct should **never be used for production purposes.**
 #[derive(Clone)]
-pub struct TestingBeaconStateBuilder {
-    state: BeaconState,
+pub struct TestingBeaconStateBuilder<T: BeaconStateTypes> {
+    state: BeaconState<T>,
     keypairs: Vec<Keypair>,
 }
 
-impl TestingBeaconStateBuilder {
+impl<T: BeaconStateTypes> TestingBeaconStateBuilder<T> {
     /// Attempts to load validators from a file in `$HOME/.lighthouse/keypairs.raw_keypairs`. If
     /// the file is unavailable, it generates the keys at runtime.
     ///
@@ -154,7 +154,7 @@ impl TestingBeaconStateBuilder {
     }
 
     /// Consume the builder and return the `BeaconState` and the keypairs for each validator.
-    pub fn build(self) -> (BeaconState, Vec<Keypair>) {
+    pub fn build(self) -> (BeaconState<T>, Vec<Keypair>) {
         (self.state, self.keypairs)
     }
 
diff --git a/eth2/types/src/test_utils/testing_pending_attestation_builder.rs b/eth2/types/src/test_utils/testing_pending_attestation_builder.rs
index 655b3d1e8..330203448 100644
--- a/eth2/types/src/test_utils/testing_pending_attestation_builder.rs
+++ b/eth2/types/src/test_utils/testing_pending_attestation_builder.rs
@@ -16,7 +16,12 @@ impl TestingPendingAttestationBuilder {
     ///
     /// * The aggregation and custody bitfields will all be empty, they need to be set with
     /// `Self::add_committee_participation`.
-    pub fn new(state: &BeaconState, shard: u64, slot: Slot, spec: &ChainSpec) -> Self {
+    pub fn new<T: BeaconStateTypes>(
+        state: &BeaconState<T>,
+        shard: u64,
+        slot: Slot,
+        spec: &ChainSpec,
+    ) -> Self {
         let data_builder = TestingAttestationDataBuilder::new(state, shard, slot, spec);
 
         let pending_attestation = PendingAttestation {
diff --git a/eth2/types/src/transfer.rs b/eth2/types/src/transfer.rs
index aea13fdd7..176a9d75d 100644
--- a/eth2/types/src/transfer.rs
+++ b/eth2/types/src/transfer.rs
@@ -2,7 +2,7 @@ use super::Slot;
 use crate::test_utils::TestRandom;
 use bls::{PublicKey, Signature};
 use derivative::Derivative;
-use rand::RngCore;
+
 use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode};
 use test_random_derive::TestRandom;
diff --git a/eth2/types/src/tree_hash_vector.rs b/eth2/types/src/tree_hash_vector.rs
index 42a730f25..2600ff62f 100644
--- a/eth2/types/src/tree_hash_vector.rs
+++ b/eth2/types/src/tree_hash_vector.rs
@@ -100,11 +100,11 @@ where
     }
 }
 
-impl<T: RngCore, U> TestRandom<T> for TreeHashVector<U>
+impl<U> TestRandom for TreeHashVector<U>
 where
-    U: TestRandom<T>,
+    U: TestRandom,
 {
-    fn random_for_test(rng: &mut T) -> Self {
+    fn random_for_test(rng: &mut impl RngCore) -> Self {
         TreeHashVector::from(vec![
             U::random_for_test(rng),
             U::random_for_test(rng),
diff --git a/eth2/types/src/validator.rs b/eth2/types/src/validator.rs
index a20eb6426..ff4cabf35 100644
--- a/eth2/types/src/validator.rs
+++ b/eth2/types/src/validator.rs
@@ -1,5 +1,5 @@
 use crate::{test_utils::TestRandom, Epoch, Hash256, PublicKey};
-use rand::RngCore;
+
 use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode};
 use test_random_derive::TestRandom;
diff --git a/eth2/types/src/voluntary_exit.rs b/eth2/types/src/voluntary_exit.rs
index 8a780db75..a138fb480 100644
--- a/eth2/types/src/voluntary_exit.rs
+++ b/eth2/types/src/voluntary_exit.rs
@@ -1,6 +1,6 @@
 use crate::{test_utils::TestRandom, Epoch};
 use bls::Signature;
-use rand::RngCore;
+
 use serde_derive::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode};
 use test_random_derive::TestRandom;
diff --git a/eth2/utils/fixed_len_vec/Cargo.toml b/eth2/utils/fixed_len_vec/Cargo.toml
new file mode 100644
index 000000000..ddfc33103
--- /dev/null
+++ b/eth2/utils/fixed_len_vec/Cargo.toml
@@ -0,0 +1,13 @@
+[package]
+name = "fixed_len_vec"
+version = "0.1.0"
+authors = ["Paul Hauner <paul@paulhauner.com>"]
+edition = "2018"
+
+[dependencies]
+cached_tree_hash = { path = "../cached_tree_hash" }
+tree_hash = { path = "../tree_hash" }
+serde = "1.0"
+serde_derive = "1.0"
+ssz = { path = "../ssz" }
+typenum = "1.10"
diff --git a/eth2/utils/fixed_len_vec/src/impls.rs b/eth2/utils/fixed_len_vec/src/impls.rs
new file mode 100644
index 000000000..98c70e3eb
--- /dev/null
+++ b/eth2/utils/fixed_len_vec/src/impls.rs
@@ -0,0 +1,70 @@
+use super::*;
+// use cached_tree_hash::CachedTreeHash;
+// use ssz::{Decodable, Encodable};
+// use tree_hash::TreeHash;
+
+impl<T, N: Unsigned> tree_hash::TreeHash for FixedLenVec<T, N>
+where
+    T: tree_hash::TreeHash,
+{
+    fn tree_hash_type() -> tree_hash::TreeHashType {
+        tree_hash::TreeHashType::Vector
+    }
+
+    fn tree_hash_packed_encoding(&self) -> Vec<u8> {
+        unreachable!("Vector should never be packed.")
+    }
+
+    fn tree_hash_packing_factor() -> usize {
+        unreachable!("Vector should never be packed.")
+    }
+
+    fn tree_hash_root(&self) -> Vec<u8> {
+        tree_hash::impls::vec_tree_hash_root(&self.vec)
+    }
+}
+
+impl<T, N: Unsigned> cached_tree_hash::CachedTreeHash for FixedLenVec<T, N>
+where
+    T: cached_tree_hash::CachedTreeHash + tree_hash::TreeHash,
+{
+    fn new_tree_hash_cache(
+        &self,
+        depth: usize,
+    ) -> Result<cached_tree_hash::TreeHashCache, cached_tree_hash::Error> {
+        let (cache, _overlay) = cached_tree_hash::vec::new_tree_hash_cache(&self.vec, depth)?;
+
+        Ok(cache)
+    }
+
+    fn tree_hash_cache_schema(&self, depth: usize) -> cached_tree_hash::BTreeSchema {
+        cached_tree_hash::vec::produce_schema(&self.vec, depth)
+    }
+
+    fn update_tree_hash_cache(
+        &self,
+        cache: &mut cached_tree_hash::TreeHashCache,
+    ) -> Result<(), cached_tree_hash::Error> {
+        cached_tree_hash::vec::update_tree_hash_cache(&self.vec, cache)?;
+
+        Ok(())
+    }
+}
+
+impl<T, N: Unsigned> ssz::Encodable for FixedLenVec<T, N>
+where
+    T: ssz::Encodable,
+{
+    fn ssz_append(&self, s: &mut ssz::SszStream) {
+        s.append_vec(&self.vec)
+    }
+}
+
+impl<T, N: Unsigned> ssz::Decodable for FixedLenVec<T, N>
+where
+    T: ssz::Decodable + Default,
+{
+    fn ssz_decode(bytes: &[u8], index: usize) -> Result<(Self, usize), ssz::DecodeError> {
+        ssz::decode_ssz_list(bytes, index).and_then(|(vec, i)| Ok((vec.into(), i)))
+    }
+}
diff --git a/eth2/types/src/beacon_state/fixed_params.rs b/eth2/utils/fixed_len_vec/src/lib.rs
similarity index 51%
rename from eth2/types/src/beacon_state/fixed_params.rs
rename to eth2/utils/fixed_len_vec/src/lib.rs
index 08aa675cf..a2e73a338 100644
--- a/eth2/types/src/beacon_state/fixed_params.rs
+++ b/eth2/utils/fixed_len_vec/src/lib.rs
@@ -1,9 +1,14 @@
-use std::borrow::{Borrow, BorrowMut};
+use serde_derive::{Deserialize, Serialize};
 use std::marker::PhantomData;
-use std::ops::{Deref, Index, IndexMut};
+use std::ops::{Index, IndexMut};
 use std::slice::SliceIndex;
 use typenum::Unsigned;
 
+pub use typenum;
+
+mod impls;
+
+#[derive(Debug, PartialEq, Clone, Serialize, Deserialize)]
 pub struct FixedLenVec<T, N>
 where
     N: Unsigned,
@@ -14,6 +19,7 @@ where
 
 impl<T: Default, N: Unsigned> From<Vec<T>> for FixedLenVec<T, N> {
     fn from(mut vec: Vec<T>) -> Self {
+        dbg!(Self::capacity());
         vec.resize_with(Self::capacity(), Default::default);
 
         Self {
@@ -23,6 +29,21 @@ impl<T: Default, N: Unsigned> From<Vec<T>> for FixedLenVec<T, N> {
     }
 }
 
+impl<T, N: Unsigned> Into<Vec<T>> for FixedLenVec<T, N> {
+    fn into(self) -> Vec<T> {
+        self.vec
+    }
+}
+
+impl<T, N: Unsigned> Default for FixedLenVec<T, N> {
+    fn default() -> Self {
+        Self {
+            vec: Vec::default(),
+            _phantom: PhantomData,
+        }
+    }
+}
+
 impl<T, N: Unsigned> FixedLenVec<T, N> {
     pub fn capacity() -> usize {
         N::to_usize()
@@ -48,25 +69,43 @@ impl<T, N: Unsigned, I: SliceIndex<[T]>> IndexMut<I> for FixedLenVec<T, N> {
 #[cfg(test)]
 mod test {
     use super::*;
-    use typenum::U8192;
+    use typenum::*;
 
     #[test]
-    fn slice_ops() {
+    fn indexing() {
         let vec = vec![1, 2];
 
         let mut fixed: FixedLenVec<u64, U8192> = vec.clone().into();
 
         assert_eq!(fixed[0], 1);
         assert_eq!(&fixed[0..1], &vec[0..1]);
-        assert_eq!(&fixed[..], &vec[..]);
+        assert_eq!((&fixed[..]).len(), 8192);
 
         fixed[1] = 3;
         assert_eq!(fixed[1], 3);
     }
+
+    #[test]
+    fn length() {
+        let vec = vec![42; 5];
+        let fixed: FixedLenVec<u64, U4> = FixedLenVec::from(vec.clone());
+        assert_eq!(&fixed[..], &vec[0..4]);
+
+        let vec = vec![42; 3];
+        let fixed: FixedLenVec<u64, U4> = FixedLenVec::from(vec.clone());
+        assert_eq!(&fixed[0..3], &vec[..]);
+        assert_eq!(&fixed[..], &vec![42, 42, 42, 0][..]);
+
+        let vec = vec![];
+        let fixed: FixedLenVec<u64, U4> = FixedLenVec::from(vec.clone());
+        assert_eq!(&fixed[..], &vec![0, 0, 0, 0][..]);
+    }
 }
 
-/*
-pub trait FixedParams {
-    type LatestCrosslinks:
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn it_works() {
+        assert_eq!(2 + 2, 4);
+    }
 }
-*/
diff --git a/eth2/utils/ssz_derive/src/lib.rs b/eth2/utils/ssz_derive/src/lib.rs
index c5b53e934..ebe04b322 100644
--- a/eth2/utils/ssz_derive/src/lib.rs
+++ b/eth2/utils/ssz_derive/src/lib.rs
@@ -100,6 +100,7 @@ pub fn ssz_encode_derive(input: TokenStream) -> TokenStream {
     let item = parse_macro_input!(input as DeriveInput);
 
     let name = &item.ident;
+    let (impl_generics, ty_generics, where_clause) = &item.generics.split_for_impl();
 
     let struct_data = match &item.data {
         syn::Data::Struct(s) => s,
@@ -109,7 +110,7 @@ pub fn ssz_encode_derive(input: TokenStream) -> TokenStream {
     let field_idents = get_serializable_named_field_idents(&struct_data);
 
     let output = quote! {
-        impl ssz::Encodable for #name {
+        impl #impl_generics ssz::Encodable for #name #ty_generics #where_clause {
             fn ssz_append(&self, s: &mut ssz::SszStream) {
                 #(
                     s.append(&self.#field_idents);
@@ -140,6 +141,7 @@ pub fn ssz_decode_derive(input: TokenStream) -> TokenStream {
     let item = parse_macro_input!(input as DeriveInput);
 
     let name = &item.ident;
+    let (impl_generics, ty_generics, where_clause) = &item.generics.split_for_impl();
 
     let struct_data = match &item.data {
         syn::Data::Struct(s) => s,
@@ -169,7 +171,7 @@ pub fn ssz_decode_derive(input: TokenStream) -> TokenStream {
     }
 
     let output = quote! {
-        impl ssz::Decodable for #name {
+        impl #impl_generics ssz::Decodable for #name #ty_generics #where_clause {
             fn ssz_decode(bytes: &[u8], i: usize) -> Result<(Self, usize), ssz::DecodeError> {
                 #(
                     #quotes
diff --git a/eth2/utils/test_random_derive/src/lib.rs b/eth2/utils/test_random_derive/src/lib.rs
index 7920ea695..a268161dd 100644
--- a/eth2/utils/test_random_derive/src/lib.rs
+++ b/eth2/utils/test_random_derive/src/lib.rs
@@ -21,6 +21,7 @@ fn should_use_default(field: &syn::Field) -> bool {
 pub fn test_random_derive(input: TokenStream) -> TokenStream {
     let derived_input = parse_macro_input!(input as DeriveInput);
     let name = &derived_input.ident;
+    let (impl_generics, ty_generics, where_clause) = &derived_input.generics.split_for_impl();
 
     let struct_data = match &derived_input.data {
         syn::Data::Struct(s) => s,
@@ -48,8 +49,8 @@ pub fn test_random_derive(input: TokenStream) -> TokenStream {
     }
 
     let output = quote! {
-        impl<T: RngCore> TestRandom<T> for #name {
-            fn random_for_test(rng: &mut T) -> Self {
+        impl #impl_generics TestRandom for #name #ty_generics #where_clause {
+            fn random_for_test(rng: &mut impl rand::RngCore) -> Self {
                Self {
                     #(
                         #quotes
diff --git a/eth2/utils/tree_hash_derive/src/lib.rs b/eth2/utils/tree_hash_derive/src/lib.rs
index 50727a89f..fe94af181 100644
--- a/eth2/utils/tree_hash_derive/src/lib.rs
+++ b/eth2/utils/tree_hash_derive/src/lib.rs
@@ -43,6 +43,7 @@ fn should_skip_hashing(field: &syn::Field) -> bool {
 #[proc_macro_derive(CachedTreeHash, attributes(tree_hash))]
 pub fn subtree_derive(input: TokenStream) -> TokenStream {
     let item = parse_macro_input!(input as DeriveInput);
+    let (impl_generics, ty_generics, where_clause) = &item.generics.split_for_impl();
 
     let name = &item.ident;
 
@@ -56,7 +57,7 @@ pub fn subtree_derive(input: TokenStream) -> TokenStream {
     let idents_c = idents_a.clone();
 
     let output = quote! {
-        impl cached_tree_hash::CachedTreeHash for #name {
+        impl #impl_generics cached_tree_hash::CachedTreeHash for #name #ty_generics #where_clause {
             fn new_tree_hash_cache(&self, depth: usize) -> Result<cached_tree_hash::TreeHashCache, cached_tree_hash::Error> {
                 let tree = cached_tree_hash::TreeHashCache::from_subtrees(
                     self,
@@ -119,6 +120,7 @@ pub fn tree_hash_derive(input: TokenStream) -> TokenStream {
     let item = parse_macro_input!(input as DeriveInput);
 
     let name = &item.ident;
+    let (impl_generics, ty_generics, where_clause) = &item.generics.split_for_impl();
 
     let struct_data = match &item.data {
         syn::Data::Struct(s) => s,
@@ -128,7 +130,7 @@ pub fn tree_hash_derive(input: TokenStream) -> TokenStream {
     let idents = get_hashable_named_field_idents(&struct_data);
 
     let output = quote! {
-        impl tree_hash::TreeHash for #name {
+        impl #impl_generics tree_hash::TreeHash for #name #ty_generics #where_clause {
             fn tree_hash_type() -> tree_hash::TreeHashType {
                 tree_hash::TreeHashType::Container
             }