diff --git a/beacon_node/beacon_chain/src/beacon_chain.rs b/beacon_node/beacon_chain/src/beacon_chain.rs
index e6fd2a134..e33912f7a 100644
--- a/beacon_node/beacon_chain/src/beacon_chain.rs
+++ b/beacon_node/beacon_chain/src/beacon_chain.rs
@@ -312,7 +312,7 @@ where
             .state
             .read()
             .get_block_root(
-                justified_epoch.start_slot(self.spec.epoch_length),
+                justified_epoch.start_slot(self.spec.slots_per_epoch),
                 &self.spec,
             )
             .ok_or_else(|| Error::BadRecentBlockRoots)?;
@@ -333,7 +333,7 @@ where
             epoch_boundary_root,
             shard_block_root: Hash256::zero(),
             latest_crosslink: Crosslink {
-                epoch: self.state.read().slot.epoch(self.spec.epoch_length),
+                epoch: self.state.read().slot.epoch(self.spec.slots_per_epoch),
                 shard_block_root: Hash256::zero(),
             },
             justified_epoch,
diff --git a/beacon_node/beacon_chain/test_harness/README.md b/beacon_node/beacon_chain/test_harness/README.md
index 12cbbe062..9dfd90d60 100644
--- a/beacon_node/beacon_chain/test_harness/README.md
+++ b/beacon_node/beacon_chain/test_harness/README.md
@@ -27,7 +27,7 @@ fork: tchaikovsky
 version: 1.0
 test_cases:
   - config:
-      epoch_length: 64
+      slots_per_epoch: 64
       deposits_for_chain_start: 1000
       num_slots: 64
       skip_slots: [2, 3]
diff --git a/beacon_node/beacon_chain/test_harness/benches/state_transition.rs b/beacon_node/beacon_chain/test_harness/benches/state_transition.rs
index aa2a858fa..7d1c44653 100644
--- a/beacon_node/beacon_chain/test_harness/benches/state_transition.rs
+++ b/beacon_node/beacon_chain/test_harness/benches/state_transition.rs
@@ -11,7 +11,7 @@ fn mid_epoch_state_transition(c: &mut Criterion) {
     let validator_count = 1000;
     let mut rig = BeaconChainHarness::new(ChainSpec::foundation(), validator_count);
 
-    let epoch_depth = (rig.spec.epoch_length * 2) + (rig.spec.epoch_length / 2);
+    let epoch_depth = (rig.spec.slots_per_epoch * 2) + (rig.spec.slots_per_epoch / 2);
 
     for _ in 0..epoch_depth {
         rig.advance_chain_with_block();
@@ -19,7 +19,7 @@ fn mid_epoch_state_transition(c: &mut Criterion) {
 
     let state = rig.beacon_chain.state.read().clone();
 
-    assert!((state.slot + 1) % rig.spec.epoch_length != 0);
+    assert!((state.slot + 1) % rig.spec.slots_per_epoch != 0);
 
     c.bench_function("mid-epoch state transition 10k validators", move |b| {
         let state = state.clone();
@@ -36,7 +36,7 @@ fn epoch_boundary_state_transition(c: &mut Criterion) {
     let validator_count = 10000;
     let mut rig = BeaconChainHarness::new(ChainSpec::foundation(), validator_count);
 
-    let epoch_depth = rig.spec.epoch_length * 2;
+    let epoch_depth = rig.spec.slots_per_epoch * 2;
 
     for _ in 0..(epoch_depth - 1) {
         rig.advance_chain_with_block();
@@ -44,7 +44,7 @@ fn epoch_boundary_state_transition(c: &mut Criterion) {
 
     let state = rig.beacon_chain.state.read().clone();
 
-    assert_eq!((state.slot + 1) % rig.spec.epoch_length, 0);
+    assert_eq!((state.slot + 1) % rig.spec.slots_per_epoch, 0);
 
     c.bench(
         "routines",
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 b7fdda9bf..56dd66558 100644
--- a/beacon_node/beacon_chain/test_harness/specs/validator_registry.yaml
+++ b/beacon_node/beacon_chain/test_harness/specs/validator_registry.yaml
@@ -5,7 +5,7 @@ fork: tchaikovsky
 version: 1.0
 test_cases:
   - config:
-      epoch_length: 64
+      slots_per_epoch: 64
       deposits_for_chain_start: 1000
       num_slots: 64
       skip_slots: [2, 3]
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 2f375f7fa..ed5fddabf 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
@@ -125,13 +125,13 @@ impl BeaconChainHarness {
 
         let nth_slot = slot
             - slot
-                .epoch(self.spec.epoch_length)
-                .start_slot(self.spec.epoch_length);
-        let nth_epoch = slot.epoch(self.spec.epoch_length) - self.spec.genesis_epoch;
+                .epoch(self.spec.slots_per_epoch)
+                .start_slot(self.spec.slots_per_epoch);
+        let nth_epoch = slot.epoch(self.spec.slots_per_epoch) - self.spec.genesis_epoch;
         debug!(
             "Advancing BeaconChain to slot {}, epoch {} (epoch height: {}, slot {} in epoch.).",
             slot,
-            slot.epoch(self.spec.epoch_length),
+            slot.epoch(self.spec.slots_per_epoch),
             nth_epoch,
             nth_slot
         );
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 8c88ee5d1..440c76cd2 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
@@ -13,7 +13,7 @@ pub struct Config {
     /// Initial validators.
     pub deposits_for_chain_start: usize,
     /// Number of slots in an epoch.
-    pub epoch_length: Option<u64>,
+    pub slots_per_epoch: Option<u64>,
     /// Number of slots to build before ending execution.
     pub num_slots: u64,
     /// Number of slots that should be skipped due to inactive validator.
@@ -34,7 +34,7 @@ impl Config {
         Self {
             deposits_for_chain_start: as_usize(&yaml, "deposits_for_chain_start")
                 .expect("Must specify validator count"),
-            epoch_length: as_u64(&yaml, "epoch_length"),
+            slots_per_epoch: as_u64(&yaml, "slots_per_epoch"),
             num_slots: as_u64(&yaml, "num_slots").expect("Must specify `config.num_slots`"),
             skip_slots: as_vec_u64(yaml, "skip_slots"),
             deposits: parse_deposits(&yaml),
diff --git a/beacon_node/beacon_chain/test_harness/src/test_case/mod.rs b/beacon_node/beacon_chain/test_harness/src/test_case/mod.rs
index f6d8d42e8..ae438a5d5 100644
--- a/beacon_node/beacon_chain/test_harness/src/test_case/mod.rs
+++ b/beacon_node/beacon_chain/test_harness/src/test_case/mod.rs
@@ -54,12 +54,12 @@ impl TestCase {
 
     /// Return a `ChainSpec::foundation()`.
     ///
-    /// If specified in `config`, returns it with a modified `epoch_length`.
+    /// If specified in `config`, returns it with a modified `slots_per_epoch`.
     fn spec(&self) -> ChainSpec {
         let mut spec = ChainSpec::foundation();
 
-        if let Some(n) = self.config.epoch_length {
-            spec.epoch_length = n;
+        if let Some(n) = self.config.slots_per_epoch {
+            spec.slots_per_epoch = n;
         }
 
         spec
@@ -174,7 +174,7 @@ impl TestCase {
 
                 for state_check in state_checks {
                     let adjusted_state_slot =
-                        state.slot - spec.genesis_epoch.start_slot(spec.epoch_length);
+                        state.slot - spec.genesis_epoch.start_slot(spec.slots_per_epoch);
 
                     if state_check.slot == adjusted_state_slot {
                         state_check.assert_valid(state, spec);
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 90c622894..3126dcfea 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
@@ -36,13 +36,13 @@ impl StateCheck {
     ///
     /// Panics with an error message if any test fails.
     pub fn assert_valid(&self, state: &BeaconState, spec: &ChainSpec) {
-        let state_epoch = state.slot.epoch(spec.epoch_length);
+        let state_epoch = state.slot.epoch(spec.slots_per_epoch);
 
         info!("Running state check for slot height {}.", self.slot);
 
         assert_eq!(
             self.slot,
-            state.slot - spec.genesis_epoch.start_slot(spec.epoch_length),
+            state.slot - spec.genesis_epoch.start_slot(spec.slots_per_epoch),
             "State slot is invalid."
         );
 
diff --git a/beacon_node/beacon_chain/test_harness/tests/chain.rs b/beacon_node/beacon_chain/test_harness/tests/chain.rs
index 238e567ad..e72c3a5aa 100644
--- a/beacon_node/beacon_chain/test_harness/tests/chain.rs
+++ b/beacon_node/beacon_chain/test_harness/tests/chain.rs
@@ -29,7 +29,7 @@ fn it_can_produce_past_first_epoch_boundary() {
 
     debug!("Harness built, tests starting..");
 
-    let blocks = harness.spec.epoch_length * 2 + 1;
+    let blocks = harness.spec.slots_per_epoch * 2 + 1;
 
     for i in 0..blocks {
         harness.advance_chain_with_block();
diff --git a/eth2/attester/src/lib.rs b/eth2/attester/src/lib.rs
index 13a1d72bb..5bffd0c50 100644
--- a/eth2/attester/src/lib.rs
+++ b/eth2/attester/src/lib.rs
@@ -195,9 +195,9 @@ mod tests {
         let beacon_node = Arc::new(SimulatedBeaconNode::default());
         let signer = Arc::new(LocalSigner::new(Keypair::random()));
 
-        let mut duties = EpochMap::new(spec.epoch_length);
+        let mut duties = EpochMap::new(spec.slots_per_epoch);
         let attest_slot = Slot::new(100);
-        let attest_epoch = attest_slot / spec.epoch_length;
+        let attest_epoch = attest_slot / spec.slots_per_epoch;
         let attest_shard = 12;
         duties.insert_attestation_shard(attest_slot, attest_shard);
         duties.set_validator_index(Some(2));
@@ -243,7 +243,7 @@ mod tests {
         );
 
         // In an epoch without known duties...
-        let slot = (attest_epoch + 1) * spec.epoch_length;
+        let slot = (attest_epoch + 1) * spec.slots_per_epoch;
         slot_clock.set_slot(slot.into());
         assert_eq!(
             attester.poll(),
diff --git a/eth2/attester/src/test_utils/epoch_map.rs b/eth2/attester/src/test_utils/epoch_map.rs
index f0dc4312e..0b5827d64 100644
--- a/eth2/attester/src/test_utils/epoch_map.rs
+++ b/eth2/attester/src/test_utils/epoch_map.rs
@@ -3,22 +3,22 @@ use std::collections::HashMap;
 use types::{Epoch, Slot};
 
 pub struct EpochMap {
-    epoch_length: u64,
+    slots_per_epoch: u64,
     validator_index: Option<u64>,
     map: HashMap<Epoch, (Slot, u64)>,
 }
 
 impl EpochMap {
-    pub fn new(epoch_length: u64) -> Self {
+    pub fn new(slots_per_epoch: u64) -> Self {
         Self {
-            epoch_length,
+            slots_per_epoch,
             validator_index: None,
             map: HashMap::new(),
         }
     }
 
     pub fn insert_attestation_shard(&mut self, slot: Slot, shard: u64) {
-        let epoch = slot.epoch(self.epoch_length);
+        let epoch = slot.epoch(self.slots_per_epoch);
         self.map.insert(epoch, (slot, shard));
     }
 
@@ -29,7 +29,7 @@ impl EpochMap {
 
 impl DutiesReader for EpochMap {
     fn attestation_shard(&self, slot: Slot) -> Result<Option<u64>, DutiesReaderError> {
-        let epoch = slot.epoch(self.epoch_length);
+        let epoch = slot.epoch(self.slots_per_epoch);
 
         match self.map.get(&epoch) {
             Some((attest_slot, attest_shard)) if *attest_slot == slot => Ok(Some(*attest_shard)),
diff --git a/eth2/block_proposer/src/lib.rs b/eth2/block_proposer/src/lib.rs
index 0e66bdc70..cea855627 100644
--- a/eth2/block_proposer/src/lib.rs
+++ b/eth2/block_proposer/src/lib.rs
@@ -132,7 +132,7 @@ impl<T: SlotClock, U: BeaconNode, V: DutiesReader, W: Signer> BlockProducer<T, U
     fn produce_block(&mut self, slot: Slot) -> Result<PollOutcome, Error> {
         let randao_reveal = {
             // TODO: add domain, etc to this message. Also ensure result matches `into_to_bytes32`.
-            let message = int_to_bytes32(slot.epoch(self.spec.epoch_length).as_u64());
+            let message = int_to_bytes32(slot.epoch(self.spec.slots_per_epoch).as_u64());
 
             match self
                 .signer
@@ -233,9 +233,9 @@ mod tests {
         let beacon_node = Arc::new(SimulatedBeaconNode::default());
         let signer = Arc::new(LocalSigner::new(Keypair::random()));
 
-        let mut epoch_map = EpochMap::new(spec.epoch_length);
+        let mut epoch_map = EpochMap::new(spec.slots_per_epoch);
         let produce_slot = Slot::new(100);
-        let produce_epoch = produce_slot.epoch(spec.epoch_length);
+        let produce_epoch = produce_slot.epoch(spec.slots_per_epoch);
         epoch_map.map.insert(produce_epoch, produce_slot);
         let epoch_map = Arc::new(epoch_map);
 
@@ -280,7 +280,7 @@ mod tests {
         );
 
         // In an epoch without known duties...
-        let slot = (produce_epoch.as_u64() + 1) * spec.epoch_length;
+        let slot = (produce_epoch.as_u64() + 1) * spec.slots_per_epoch;
         slot_clock.set_slot(slot);
         assert_eq!(
             block_proposer.poll(),
diff --git a/eth2/block_proposer/src/test_utils/epoch_map.rs b/eth2/block_proposer/src/test_utils/epoch_map.rs
index e9ed9b68a..f7d8dcdcb 100644
--- a/eth2/block_proposer/src/test_utils/epoch_map.rs
+++ b/eth2/block_proposer/src/test_utils/epoch_map.rs
@@ -3,14 +3,14 @@ use std::collections::HashMap;
 use types::{Epoch, Slot};
 
 pub struct EpochMap {
-    epoch_length: u64,
+    slots_per_epoch: u64,
     pub map: HashMap<Epoch, Slot>,
 }
 
 impl EpochMap {
-    pub fn new(epoch_length: u64) -> Self {
+    pub fn new(slots_per_epoch: u64) -> Self {
         Self {
-            epoch_length,
+            slots_per_epoch,
             map: HashMap::new(),
         }
     }
@@ -18,7 +18,7 @@ impl EpochMap {
 
 impl DutiesReader for EpochMap {
     fn is_block_production_slot(&self, slot: Slot) -> Result<bool, DutiesReaderError> {
-        let epoch = slot.epoch(self.epoch_length);
+        let epoch = slot.epoch(self.slots_per_epoch);
         match self.map.get(&epoch) {
             Some(s) if *s == slot => Ok(true),
             Some(s) if *s != slot => Ok(false),
diff --git a/eth2/fork_choice/src/bitwise_lmd_ghost.rs b/eth2/fork_choice/src/bitwise_lmd_ghost.rs
index 60aa38fe7..c13aacbf4 100644
--- a/eth2/fork_choice/src/bitwise_lmd_ghost.rs
+++ b/eth2/fork_choice/src/bitwise_lmd_ghost.rs
@@ -95,7 +95,7 @@ where
 
         let active_validator_indices = get_active_validator_indices(
             &current_state.validator_registry[..],
-            block_slot.epoch(spec.epoch_length),
+            block_slot.epoch(spec.slots_per_epoch),
         );
 
         for index in active_validator_indices {
diff --git a/eth2/fork_choice/src/slow_lmd_ghost.rs b/eth2/fork_choice/src/slow_lmd_ghost.rs
index 3aafb3924..ab4cd2ada 100644
--- a/eth2/fork_choice/src/slow_lmd_ghost.rs
+++ b/eth2/fork_choice/src/slow_lmd_ghost.rs
@@ -64,7 +64,7 @@ where
 
         let active_validator_indices = get_active_validator_indices(
             &current_state.validator_registry[..],
-            block_slot.epoch(spec.epoch_length),
+            block_slot.epoch(spec.slots_per_epoch),
         );
 
         for index in active_validator_indices {
diff --git a/eth2/state_processing/src/block_processable.rs b/eth2/state_processing/src/block_processable.rs
index 32327aad3..ee8bc7c33 100644
--- a/eth2/state_processing/src/block_processable.rs
+++ b/eth2/state_processing/src/block_processable.rs
@@ -185,7 +185,7 @@ fn per_block_processing_signature_optional(
                     proposer_slashing
                         .proposal_data_1
                         .slot
-                        .epoch(spec.epoch_length),
+                        .epoch(spec.slots_per_epoch),
                     spec.domain_proposal
                 )
             ),
@@ -201,7 +201,7 @@ fn per_block_processing_signature_optional(
                     proposer_slashing
                         .proposal_data_2
                         .slot
-                        .epoch(spec.epoch_length),
+                        .epoch(spec.slots_per_epoch),
                     spec.domain_proposal
                 )
             ),
@@ -341,14 +341,14 @@ fn validate_attestation_signature_optional(
 ) -> Result<(), AttestationValidationError> {
     trace!(
         "validate_attestation_signature_optional: attestation epoch: {}",
-        attestation.data.slot.epoch(spec.epoch_length)
+        attestation.data.slot.epoch(spec.slots_per_epoch)
     );
     ensure!(
         attestation.data.slot + spec.min_attestation_inclusion_delay <= state.slot,
         AttestationValidationError::IncludedTooEarly
     );
     ensure!(
-        attestation.data.slot + spec.epoch_length >= state.slot,
+        attestation.data.slot + spec.slots_per_epoch >= state.slot,
         AttestationValidationError::IncludedTooLate
     );
     if attestation.data.slot >= state.current_epoch_start_slot(spec) {
@@ -369,7 +369,7 @@ fn validate_attestation_signature_optional(
                     attestation
                         .data
                         .justified_epoch
-                        .start_slot(spec.epoch_length),
+                        .start_slot(spec.slots_per_epoch),
                     &spec
                 )
                 .ok_or(AttestationValidationError::NoBlockRoot)?,
@@ -377,7 +377,7 @@ fn validate_attestation_signature_optional(
     );
     let potential_crosslink = Crosslink {
         shard_block_root: attestation.data.shard_block_root,
-        epoch: attestation.data.slot.epoch(spec.epoch_length),
+        epoch: attestation.data.slot.epoch(spec.slots_per_epoch),
     };
     ensure!(
         (attestation.data.latest_crosslink
@@ -407,7 +407,7 @@ fn validate_attestation_signature_optional(
                 PHASE_0_CUSTODY_BIT,
                 get_domain(
                     &state.fork,
-                    attestation.data.slot.epoch(spec.epoch_length),
+                    attestation.data.slot.epoch(spec.slots_per_epoch),
                     spec.domain_attestation,
                 )
             ),
diff --git a/eth2/state_processing/src/epoch_processable.rs b/eth2/state_processing/src/epoch_processable.rs
index 0ecd1bbd1..47336d3f8 100644
--- a/eth2/state_processing/src/epoch_processable.rs
+++ b/eth2/state_processing/src/epoch_processable.rs
@@ -76,7 +76,7 @@ impl EpochProcessable for BeaconState {
          */
         let active_validator_indices = get_active_validator_indices(
             &self.validator_registry,
-            self.slot.epoch(spec.epoch_length),
+            self.slot.epoch(spec.slots_per_epoch),
         );
         let current_total_balance = self.get_total_balance(&active_validator_indices[..], spec);
 
@@ -90,7 +90,7 @@ impl EpochProcessable for BeaconState {
             .latest_attestations
             .par_iter()
             .filter(|a| {
-                (a.data.slot / spec.epoch_length).epoch(spec.epoch_length)
+                (a.data.slot / spec.slots_per_epoch).epoch(spec.slots_per_epoch)
                     == self.current_epoch(spec)
             })
             .collect();
@@ -137,7 +137,7 @@ impl EpochProcessable for BeaconState {
             .par_iter()
             .filter(|a| {
                 //TODO: ensure these saturating subs are correct.
-                (a.data.slot / spec.epoch_length).epoch(spec.epoch_length)
+                (a.data.slot / spec.slots_per_epoch).epoch(spec.slots_per_epoch)
                     == self.previous_epoch(spec)
             })
             .collect();
@@ -320,12 +320,12 @@ impl EpochProcessable for BeaconState {
         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) {
+        // for slot in self.slot.saturating_sub(2 * spec.slots_per_epoch)..self.slot {
+        for slot in self.previous_epoch(spec).slot_iter(spec.slots_per_epoch) {
             trace!(
                 "Finding winning root for slot: {} (epoch: {})",
                 slot,
-                slot.epoch(spec.epoch_length)
+                slot.epoch(spec.slots_per_epoch)
             );
 
             // Clone is used to remove the borrow. It becomes an issue later when trying to mutate
@@ -506,7 +506,7 @@ impl EpochProcessable for BeaconState {
         /*
          * Crosslinks
          */
-        for slot in self.previous_epoch(spec).slot_iter(spec.epoch_length) {
+        for slot in self.previous_epoch(spec).slot_iter(spec.slots_per_epoch) {
             // Clone is used to remove the borrow. It becomes an issue later when trying to mutate
             // `self.balances`.
             let crosslink_committees_at_slot =
@@ -615,7 +615,7 @@ impl EpochProcessable for BeaconState {
         self.latest_attestations = self
             .latest_attestations
             .iter()
-            .filter(|a| a.data.slot.epoch(spec.epoch_length) >= current_epoch)
+            .filter(|a| a.data.slot.epoch(spec.slots_per_epoch) >= current_epoch)
             .cloned()
             .collect();
 
diff --git a/eth2/state_processing/src/slot_processable.rs b/eth2/state_processing/src/slot_processable.rs
index 6017f4c0a..8d6506c36 100644
--- a/eth2/state_processing/src/slot_processable.rs
+++ b/eth2/state_processing/src/slot_processable.rs
@@ -24,7 +24,7 @@ where
         previous_block_root: Hash256,
         spec: &ChainSpec,
     ) -> Result<(), Error> {
-        if (self.slot + 1) % spec.epoch_length == 0 {
+        if (self.slot + 1) % spec.slots_per_epoch == 0 {
             self.per_epoch_processing(spec)?;
             self.advance_caches();
         }
diff --git a/eth2/types/src/beacon_state.rs b/eth2/types/src/beacon_state.rs
index b420f51d5..75beb79ca 100644
--- a/eth2/types/src/beacon_state.rs
+++ b/eth2/types/src/beacon_state.rs
@@ -337,7 +337,7 @@ impl BeaconState {
     ///
     /// Spec v0.2.0
     pub fn current_epoch(&self, spec: &ChainSpec) -> Epoch {
-        self.slot.epoch(spec.epoch_length)
+        self.slot.epoch(spec.slots_per_epoch)
     }
 
     /// The epoch prior to `self.current_epoch()`.
@@ -363,14 +363,14 @@ impl BeaconState {
     ///
     /// Spec v0.2.0
     pub fn current_epoch_start_slot(&self, spec: &ChainSpec) -> Slot {
-        self.current_epoch(spec).start_slot(spec.epoch_length)
+        self.current_epoch(spec).start_slot(spec.slots_per_epoch)
     }
 
     /// The first slot of the epoch preceeding the one corresponding to `self.slot`.
     ///
     /// Spec v0.2.0
     pub fn previous_epoch_start_slot(&self, spec: &ChainSpec) -> Slot {
-        self.previous_epoch(spec).start_slot(spec.epoch_length)
+        self.previous_epoch(spec).start_slot(spec.slots_per_epoch)
     }
 
     /// Return the number of committees in one epoch.
@@ -386,10 +386,10 @@ impl BeaconState {
         std::cmp::max(
             1,
             std::cmp::min(
-                spec.shard_count / spec.epoch_length,
-                active_validator_count as u64 / spec.epoch_length / spec.target_committee_size,
+                spec.shard_count / spec.slots_per_epoch,
+                active_validator_count as u64 / spec.slots_per_epoch / spec.target_committee_size,
             ),
-        ) * spec.epoch_length
+        ) * spec.slots_per_epoch
     }
 
     /// Shuffle ``validators`` into crosslink committees seeded by ``seed`` and ``epoch``.
@@ -520,11 +520,11 @@ impl BeaconState {
         slot: Slot,
         spec: &ChainSpec,
     ) -> Result<&CrosslinkCommittees, Error> {
-        let epoch = slot.epoch(spec.epoch_length);
+        let epoch = slot.epoch(spec.slots_per_epoch);
         let relative_epoch = self.relative_epoch(epoch, spec)?;
         let cache = self.cache(relative_epoch)?;
 
-        let slot_offset = slot - epoch.start_slot(spec.epoch_length);
+        let slot_offset = slot - epoch.start_slot(spec.slots_per_epoch);
 
         Ok(&cache.committees[slot_offset.as_usize()])
     }
@@ -565,7 +565,7 @@ impl BeaconState {
         registry_change: bool,
         spec: &ChainSpec,
     ) -> Result<(u64, Hash256, Epoch, u64), Error> {
-        let epoch = slot.epoch(spec.epoch_length);
+        let epoch = slot.epoch(spec.slots_per_epoch);
         let current_epoch = self.current_epoch(spec);
         let previous_epoch = self.previous_epoch(spec);
         let next_epoch = self.next_epoch(spec);
@@ -639,8 +639,8 @@ impl BeaconState {
         let (committees_per_epoch, _seed, _shuffling_epoch, shuffling_start_shard) =
             self.get_committee_params_at_slot(slot, registry_change, spec)?;
 
-        let offset = slot.as_u64() % spec.epoch_length;
-        let committees_per_slot = committees_per_epoch / spec.epoch_length;
+        let offset = slot.as_u64() % spec.slots_per_epoch;
+        let committees_per_slot = committees_per_epoch / spec.slots_per_epoch;
         let slot_start_shard =
             (shuffling_start_shard + committees_per_slot * offset) % spec.shard_count;
 
@@ -835,7 +835,7 @@ impl BeaconState {
         proof_of_possession.verify(
             &proof_of_possession_data.hash_tree_root(),
             self.fork
-                .get_domain(self.slot.epoch(spec.epoch_length), spec.domain_deposit),
+                .get_domain(self.slot.epoch(spec.slots_per_epoch), spec.domain_deposit),
             &pubkey,
         )
     }
@@ -1296,7 +1296,7 @@ impl BeaconState {
         bitfield: &Bitfield,
         spec: &ChainSpec,
     ) -> Result<Vec<usize>, Error> {
-        let epoch = attestation_data.slot.epoch(spec.epoch_length);
+        let epoch = attestation_data.slot.epoch(spec.slots_per_epoch);
         let relative_epoch = self.relative_epoch(epoch, spec)?;
         let cache = self.cache(relative_epoch)?;
 
diff --git a/eth2/types/src/beacon_state/builder.rs b/eth2/types/src/beacon_state/builder.rs
index 02886a86e..4acd172f5 100644
--- a/eth2/types/src/beacon_state/builder.rs
+++ b/eth2/types/src/beacon_state/builder.rs
@@ -137,7 +137,7 @@ impl BeaconStateBuilder {
     pub fn teleport_to_end_of_epoch(&mut self, epoch: Epoch) {
         let state = self.state.as_mut().expect("Genesis required");
 
-        let slot = epoch.end_slot(self.spec.epoch_length);
+        let slot = epoch.end_slot(self.spec.slots_per_epoch);
 
         state.slot = slot;
         state.validator_registry_update_epoch = epoch - 1;
@@ -171,11 +171,11 @@ impl BeaconStateBuilder {
         let current_epoch = state.current_epoch(&self.spec);
         let previous_epoch = state.previous_epoch(&self.spec);
         let current_epoch_depth =
-            (state.slot - current_epoch.end_slot(self.spec.epoch_length)).as_usize();
+            (state.slot - current_epoch.end_slot(self.spec.slots_per_epoch)).as_usize();
 
-        let previous_epoch_slots = previous_epoch.slot_iter(self.spec.epoch_length);
+        let previous_epoch_slots = previous_epoch.slot_iter(self.spec.slots_per_epoch);
         let current_epoch_slots = current_epoch
-            .slot_iter(self.spec.epoch_length)
+            .slot_iter(self.spec.slots_per_epoch)
             .take(current_epoch_depth);
 
         for slot in previous_epoch_slots.chain(current_epoch_slots) {
@@ -219,7 +219,8 @@ fn committee_to_pending_attestation(
         custody_bitfield.set(i, true);
     }
 
-    let is_previous_epoch = state.slot.epoch(spec.epoch_length) != slot.epoch(spec.epoch_length);
+    let is_previous_epoch =
+        state.slot.epoch(spec.slots_per_epoch) != slot.epoch(spec.slots_per_epoch);
 
     let justified_epoch = if is_previous_epoch {
         state.previous_justified_epoch
@@ -229,16 +230,16 @@ fn committee_to_pending_attestation(
 
     let epoch_boundary_root = if is_previous_epoch {
         *state
-            .get_block_root(previous_epoch.start_slot(spec.epoch_length), spec)
+            .get_block_root(previous_epoch.start_slot(spec.slots_per_epoch), spec)
             .unwrap()
     } else {
         *state
-            .get_block_root(current_epoch.start_slot(spec.epoch_length), spec)
+            .get_block_root(current_epoch.start_slot(spec.slots_per_epoch), spec)
             .unwrap()
     };
 
     let justified_block_root = *state
-        .get_block_root(justified_epoch.start_slot(spec.epoch_length), &spec)
+        .get_block_root(justified_epoch.start_slot(spec.slots_per_epoch), &spec)
         .unwrap();
 
     PendingAttestation {
@@ -250,7 +251,7 @@ fn committee_to_pending_attestation(
             epoch_boundary_root,
             shard_block_root: Hash256::zero(),
             latest_crosslink: Crosslink {
-                epoch: slot.epoch(spec.epoch_length),
+                epoch: slot.epoch(spec.slots_per_epoch),
                 shard_block_root: Hash256::zero(),
             },
             justified_epoch,
diff --git a/eth2/types/src/beacon_state/epoch_cache.rs b/eth2/types/src/beacon_state/epoch_cache.rs
index ee3a67813..bbc991646 100644
--- a/eth2/types/src/beacon_state/epoch_cache.rs
+++ b/eth2/types/src/beacon_state/epoch_cache.rs
@@ -32,14 +32,14 @@ impl EpochCache {
         spec: &ChainSpec,
     ) -> Result<EpochCache, Error> {
         let mut epoch_committees: Vec<CrosslinkCommittees> =
-            Vec::with_capacity(spec.epoch_length as usize);
+            Vec::with_capacity(spec.slots_per_epoch as usize);
         let mut attestation_duty_map: AttestationDutyMap = HashMap::new();
         let mut shard_committee_index_map: ShardCommitteeIndexMap = HashMap::new();
 
         let shuffling =
-            state.get_shuffling_for_slot(epoch.start_slot(spec.epoch_length), false, spec)?;
+            state.get_shuffling_for_slot(epoch.start_slot(spec.slots_per_epoch), false, spec)?;
 
-        for (epoch_committeess_index, slot) in epoch.slot_iter(spec.epoch_length).enumerate() {
+        for (epoch_committeess_index, slot) in epoch.slot_iter(spec.slots_per_epoch).enumerate() {
             let slot_committees = state.calculate_crosslink_committees_at_slot(
                 slot,
                 false,
diff --git a/eth2/types/src/beacon_state/tests.rs b/eth2/types/src/beacon_state/tests.rs
index bb8561511..40bfd146c 100644
--- a/eth2/types/src/beacon_state/tests.rs
+++ b/eth2/types/src/beacon_state/tests.rs
@@ -35,8 +35,8 @@ pub fn get_attestation_participants_consistency() {
 
     for slot in state
         .slot
-        .epoch(spec.epoch_length)
-        .slot_iter(spec.epoch_length)
+        .epoch(spec.slots_per_epoch)
+        .slot_iter(spec.slots_per_epoch)
     {
         let committees = state.get_crosslink_committees_at_slot(slot, &spec).unwrap();
 
diff --git a/eth2/types/src/proposer_slashing/builder.rs b/eth2/types/src/proposer_slashing/builder.rs
index 66f74a2e2..8dc1abfbe 100644
--- a/eth2/types/src/proposer_slashing/builder.rs
+++ b/eth2/types/src/proposer_slashing/builder.rs
@@ -38,14 +38,14 @@ impl ProposerSlashingBuilder {
 
         proposal_1.signature = {
             let message = proposal_1.signed_root();
-            let epoch = slot.epoch(spec.epoch_length);
+            let epoch = slot.epoch(spec.slots_per_epoch);
             let domain = spec.domain_proposal;
             signer(proposer_index, &message[..], epoch, domain)
         };
 
         proposal_2.signature = {
             let message = proposal_2.signed_root();
-            let epoch = slot.epoch(spec.epoch_length);
+            let epoch = slot.epoch(spec.slots_per_epoch);
             let domain = spec.domain_proposal;
             signer(proposer_index, &message[..], epoch, domain)
         };
diff --git a/eth2/types/src/slashable_attestation.rs b/eth2/types/src/slashable_attestation.rs
index 9f2ccab60..20ba76cdb 100644
--- a/eth2/types/src/slashable_attestation.rs
+++ b/eth2/types/src/slashable_attestation.rs
@@ -24,7 +24,7 @@ impl SlashableAttestation {
     ///
     /// Spec v0.4.0
     pub fn is_double_vote(&self, other: &SlashableAttestation, spec: &ChainSpec) -> bool {
-        self.data.slot.epoch(spec.epoch_length) == other.data.slot.epoch(spec.epoch_length)
+        self.data.slot.epoch(spec.slots_per_epoch) == other.data.slot.epoch(spec.slots_per_epoch)
     }
 
     /// Check if ``attestation_data_1`` surrounds ``attestation_data_2``.
@@ -33,8 +33,8 @@ impl SlashableAttestation {
     pub fn is_surround_vote(&self, other: &SlashableAttestation, 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);
+        let target_epoch_1 = self.data.slot.epoch(spec.slots_per_epoch);
+        let target_epoch_2 = other.data.slot.epoch(spec.slots_per_epoch);
 
         (source_epoch_1 < source_epoch_2) & (target_epoch_2 < target_epoch_1)
     }
@@ -151,7 +151,7 @@ mod tests {
         let mut rng = XorShiftRng::from_seed([42; 16]);
         let mut slashable_vote = SlashableAttestation::random_for_test(&mut rng);
 
-        slashable_vote.data.slot = Slot::new(slot_factor * spec.epoch_length);
+        slashable_vote.data.slot = Slot::new(slot_factor * spec.slots_per_epoch);
         slashable_vote.data.justified_epoch = Epoch::new(justified_epoch);
         slashable_vote
     }
diff --git a/eth2/types/src/slot_epoch.rs b/eth2/types/src/slot_epoch.rs
index ff4fd5b9b..7753027a6 100644
--- a/eth2/types/src/slot_epoch.rs
+++ b/eth2/types/src/slot_epoch.rs
@@ -35,8 +35,8 @@ impl Slot {
         Slot(slot)
     }
 
-    pub fn epoch(self, epoch_length: u64) -> Epoch {
-        Epoch::from(self.0 / epoch_length)
+    pub fn epoch(self, slots_per_epoch: u64) -> Epoch {
+        Epoch::from(self.0 / slots_per_epoch)
     }
 
     pub fn height(self, genesis_slot: Slot) -> SlotHeight {
@@ -57,24 +57,24 @@ impl Epoch {
         Epoch(u64::max_value())
     }
 
-    pub fn start_slot(self, epoch_length: u64) -> Slot {
-        Slot::from(self.0.saturating_mul(epoch_length))
+    pub fn start_slot(self, slots_per_epoch: u64) -> Slot {
+        Slot::from(self.0.saturating_mul(slots_per_epoch))
     }
 
-    pub fn end_slot(self, epoch_length: u64) -> Slot {
+    pub fn end_slot(self, slots_per_epoch: u64) -> Slot {
         Slot::from(
             self.0
                 .saturating_add(1)
-                .saturating_mul(epoch_length)
+                .saturating_mul(slots_per_epoch)
                 .saturating_sub(1),
         )
     }
 
-    pub fn slot_iter(&self, epoch_length: u64) -> SlotIter {
+    pub fn slot_iter(&self, slots_per_epoch: u64) -> SlotIter {
         SlotIter {
             current_iteration: 0,
             epoch: self,
-            epoch_length,
+            slots_per_epoch,
         }
     }
 }
@@ -82,17 +82,17 @@ impl Epoch {
 pub struct SlotIter<'a> {
     current_iteration: u64,
     epoch: &'a Epoch,
-    epoch_length: u64,
+    slots_per_epoch: u64,
 }
 
 impl<'a> Iterator for SlotIter<'a> {
     type Item = Slot;
 
     fn next(&mut self) -> Option<Slot> {
-        if self.current_iteration >= self.epoch_length {
+        if self.current_iteration >= self.slots_per_epoch {
             None
         } else {
-            let start_slot = self.epoch.start_slot(self.epoch_length);
+            let start_slot = self.epoch.start_slot(self.slots_per_epoch);
             let previous = self.current_iteration;
             self.current_iteration += 1;
             Some(start_slot + previous)
@@ -119,18 +119,18 @@ mod epoch_tests {
 
     #[test]
     fn slot_iter() {
-        let epoch_length = 8;
+        let slots_per_epoch = 8;
 
         let epoch = Epoch::new(0);
 
         let mut slots = vec![];
-        for slot in epoch.slot_iter(epoch_length) {
+        for slot in epoch.slot_iter(slots_per_epoch) {
             slots.push(slot);
         }
 
-        assert_eq!(slots.len(), epoch_length as usize);
+        assert_eq!(slots.len(), slots_per_epoch as usize);
 
-        for i in 0..epoch_length {
+        for i in 0..slots_per_epoch {
             assert_eq!(Slot::from(i), slots[i as usize])
         }
     }
diff --git a/eth2/types/src/slot_height.rs b/eth2/types/src/slot_height.rs
index f9370f485..1739227a4 100644
--- a/eth2/types/src/slot_height.rs
+++ b/eth2/types/src/slot_height.rs
@@ -23,8 +23,8 @@ impl SlotHeight {
         Slot::from(self.0.saturating_add(genesis_slot.as_u64()))
     }
 
-    pub fn epoch(self, genesis_slot: u64, epoch_length: u64) -> Epoch {
-        Epoch::from(self.0.saturating_add(genesis_slot) / epoch_length)
+    pub fn epoch(self, genesis_slot: u64, slots_per_epoch: u64) -> Epoch {
+        Epoch::from(self.0.saturating_add(genesis_slot) / slots_per_epoch)
     }
 
     pub fn max_value() -> SlotHeight {
diff --git a/validator_client/src/duties/epoch_duties.rs b/validator_client/src/duties/epoch_duties.rs
index 54a882f8d..d1bbfa156 100644
--- a/validator_client/src/duties/epoch_duties.rs
+++ b/validator_client/src/duties/epoch_duties.rs
@@ -32,14 +32,14 @@ pub enum EpochDutiesMapError {
 
 /// Maps an `epoch` to some `EpochDuties` for a single validator.
 pub struct EpochDutiesMap {
-    pub epoch_length: u64,
+    pub slots_per_epoch: u64,
     pub map: RwLock<HashMap<Epoch, EpochDuties>>,
 }
 
 impl EpochDutiesMap {
-    pub fn new(epoch_length: u64) -> Self {
+    pub fn new(slots_per_epoch: u64) -> Self {
         Self {
-            epoch_length,
+            slots_per_epoch,
             map: RwLock::new(HashMap::new()),
         }
     }
@@ -67,7 +67,7 @@ impl EpochDutiesMap {
 
 impl DutiesReader for EpochDutiesMap {
     fn is_block_production_slot(&self, slot: Slot) -> Result<bool, DutiesReaderError> {
-        let epoch = slot.epoch(self.epoch_length);
+        let epoch = slot.epoch(self.slots_per_epoch);
 
         let map = self.map.read().map_err(|_| DutiesReaderError::Poisoned)?;
         let duties = map
diff --git a/validator_client/src/duties/mod.rs b/validator_client/src/duties/mod.rs
index febab4755..29bd81d0a 100644
--- a/validator_client/src/duties/mod.rs
+++ b/validator_client/src/duties/mod.rs
@@ -61,7 +61,7 @@ impl<T: SlotClock, U: BeaconNode> DutiesManager<T, U> {
             .map_err(|_| Error::SlotClockError)?
             .ok_or(Error::SlotUnknowable)?;
 
-        let epoch = slot.epoch(self.spec.epoch_length);
+        let epoch = slot.epoch(self.spec.slots_per_epoch);
 
         if let Some(duties) = self.beacon_node.request_shuffling(epoch, &self.pubkey)? {
             // If these duties were known, check to see if they're updates or identical.
@@ -112,7 +112,7 @@ mod tests {
     #[test]
     pub fn polling() {
         let spec = Arc::new(ChainSpec::foundation());
-        let duties_map = Arc::new(EpochDutiesMap::new(spec.epoch_length));
+        let duties_map = Arc::new(EpochDutiesMap::new(spec.slots_per_epoch));
         let keypair = Keypair::random();
         let slot_clock = Arc::new(TestingSlotClock::new(0));
         let beacon_node = Arc::new(TestBeaconNode::default());
diff --git a/validator_client/src/main.rs b/validator_client/src/main.rs
index c835300b5..b4d8ae5db 100644
--- a/validator_client/src/main.rs
+++ b/validator_client/src/main.rs
@@ -111,7 +111,7 @@ fn main() {
 
     for keypair in keypairs {
         info!(log, "Starting validator services"; "validator" => keypair.pk.concatenated_hex_id());
-        let duties_map = Arc::new(EpochDutiesMap::new(spec.epoch_length));
+        let duties_map = Arc::new(EpochDutiesMap::new(spec.slots_per_epoch));
 
         // Spawn a new thread to maintain the validator's `EpochDuties`.
         let duties_manager_thread = {