diff --git a/eth2/types/src/beacon_state.rs b/eth2/types/src/beacon_state.rs
index 362460c0c..b7f869c8d 100644
--- a/eth2/types/src/beacon_state.rs
+++ b/eth2/types/src/beacon_state.rs
@@ -1,7 +1,7 @@
 use crate::test_utils::TestRandom;
 use crate::{
     validator::StatusFlags, validator_registry::get_active_validator_indices, AggregatePublicKey,
-    Attestation, AttestationData, BeaconBlock, Bitfield, ChainSpec, Crosslink, Eth1Data,
+    Attestation, AttestationData, BeaconBlock, Bitfield, ChainSpec, Crosslink, Epoch, Eth1Data,
     Eth1DataVote, Exit, Fork, Hash256, PendingAttestation, PublicKey, Signature, Slot, Validator,
 };
 use bls::bls_verify_aggregate;
@@ -28,7 +28,7 @@ const DOMAIN_ATTESTATION: u64 = 1;
 pub enum Error {
     InsufficientValidators,
     BadBlockSignature,
-    InvalidEpoch(u64, Range<u64>),
+    InvalidEpoch(Slot, Range<Epoch>),
     CommitteesError(CommitteesError),
 }
 
@@ -81,7 +81,7 @@ pub enum WinningRootError {
 
 #[derive(Debug, PartialEq)]
 pub enum CommitteesError {
-    InvalidEpoch(u64, Range<u64>),
+    InvalidEpoch,
     InsufficientNumberOfValidators,
 }
 
@@ -198,20 +198,20 @@ impl BeaconState {
         Hash256::from(&self.hash_tree_root()[..])
     }
 
-    pub fn current_epoch(&self, spec: &ChainSpec) -> u64 {
-        self.slot / spec.epoch_length
+    pub fn current_epoch(&self, spec: &ChainSpec) -> Epoch {
+        self.slot.epoch(spec.epoch_length)
     }
 
-    pub fn previous_epoch(&self, spec: &ChainSpec) -> u64 {
-        self.current_epoch(spec).saturating_sub(1)
+    pub fn previous_epoch(&self, spec: &ChainSpec) -> Epoch {
+        self.current_epoch(spec).saturating_sub(1_u64)
     }
 
-    pub fn current_epoch_start_slot(&self, spec: &ChainSpec) -> u64 {
-        self.current_epoch(spec) * spec.epoch_length
+    pub fn current_epoch_start_slot(&self, spec: &ChainSpec) -> Slot {
+        self.current_epoch(spec).start_slot(spec.epoch_length)
     }
 
-    pub fn previous_epoch_start_slot(&self, spec: &ChainSpec) -> u64 {
-        self.previous_epoch(spec) * spec.epoch_length
+    pub fn previous_epoch_start_slot(&self, spec: &ChainSpec) -> Slot {
+        self.previous_epoch(spec).start_slot(spec.epoch_length)
     }
 
     /// Returns the number of committees per slot.
@@ -231,23 +231,6 @@ impl BeaconState {
         )
     }
 
-    /// Returns the start slot and end slot of the current epoch containing `self.slot`.
-    pub fn get_current_epoch_boundaries(&self, epoch_length: u64) -> Range<u64> {
-        let slot_in_epoch = self.slot % epoch_length;
-        let start = self.slot - slot_in_epoch;
-        let end = self.slot + (epoch_length - slot_in_epoch);
-        start..end
-    }
-
-    /// Returns the start slot and end slot of the current epoch containing `self.slot`.
-    pub fn get_previous_epoch_boundaries(&self, spec: &ChainSpec) -> Range<u64> {
-        let current_epoch = self.slot / spec.epoch_length;
-        let previous_epoch = current_epoch.saturating_sub(1);
-        let start = previous_epoch * spec.epoch_length;
-        let end = start + spec.epoch_length;
-        start..end
-    }
-
     fn get_previous_epoch_committee_count_per_slot(&self, spec: &ChainSpec) -> u64 {
         let previous_active_validators = get_active_validator_indices(
             &self.validator_registry,
@@ -266,24 +249,24 @@ impl BeaconState {
 
     pub fn get_crosslink_committees_at_slot(
         &self,
-        slot: u64,
+        slot: Slot,
         spec: &ChainSpec,
     ) -> Result<Vec<(Vec<usize>, u64)>, CommitteesError> {
-        let epoch = slot / spec.epoch_length;
-        let current_epoch = self.slot / spec.epoch_length;
-        let previous_epoch = if current_epoch == spec.genesis_slot {
+        let epoch = slot.epoch(spec.epoch_length);
+        let current_epoch = self.current_epoch(spec);
+        let previous_epoch = if current_epoch == spec.genesis_slot.epoch(spec.epoch_length) {
             current_epoch
         } else {
-            current_epoch.saturating_sub(1)
+            current_epoch.saturating_sub(1_u64)
         };
         let next_epoch = current_epoch + 1;
 
         ensure!(
             (previous_epoch <= epoch) & (epoch < next_epoch),
-            CommitteesError::InvalidEpoch(slot, previous_epoch..current_epoch)
+            CommitteesError::InvalidEpoch
         );
 
-        let offset = slot % spec.epoch_length;
+        let offset = slot.as_u64() % spec.epoch_length;
 
         let (committees_per_slot, shuffling, slot_start_shard) = if epoch < current_epoch {
             let committees_per_slot = self.get_previous_epoch_committee_count_per_slot(spec);
@@ -332,11 +315,11 @@ impl BeaconState {
         let block_proposer = self.get_beacon_proposer_index(self.slot, spec)?;
 
         self.validator_registry[block_proposer].proposer_slots += 1;
-        self.latest_randao_mixes[(self.slot % spec.latest_randao_mixes_length) as usize] =
-            self.latest_randao_mixes[((self.slot - 1) % spec.latest_randao_mixes_length) as usize];
+        self.latest_randao_mixes[(self.slot % spec.latest_randao_mixes_length).as_usize()] = self
+            .latest_randao_mixes[((self.slot - 1) % spec.latest_randao_mixes_length).as_usize()];
 
         // Block roots.
-        self.latest_block_roots[((self.slot - 1) % spec.latest_block_roots_length) as usize] =
+        self.latest_block_roots[((self.slot - 1) % spec.latest_block_roots_length).as_usize()] =
             previous_block_root;
 
         if self.slot % spec.latest_block_roots_length == 0 {
@@ -350,9 +333,9 @@ impl BeaconState {
         &self,
         validator_index: usize,
         spec: &ChainSpec,
-    ) -> Result<Option<(u64, u64, u64)>, CommitteesError> {
+    ) -> Result<Option<(Slot, u64, u64)>, CommitteesError> {
         let mut result = None;
-        for slot in self.get_current_epoch_boundaries(spec.epoch_length) {
+        for slot in self.current_epoch(spec).slot_iter(spec.epoch_length) {
             for (committee, shard) in self.get_crosslink_committees_at_slot(slot, spec)? {
                 if let Some(committee_index) = committee.iter().position(|&i| i == validator_index)
                 {
@@ -426,13 +409,14 @@ impl BeaconState {
         // TODO: check this is correct.
         let new_mix = {
             let mut mix = self.latest_randao_mixes
-                [(self.slot % spec.latest_randao_mixes_length) as usize]
-                .to_vec();
+                [(self.slot % spec.latest_randao_mixes_length).as_usize()]
+            .to_vec();
             mix.append(&mut ssz_encode(&block.randao_reveal));
             Hash256::from(&hash(&mix)[..])
         };
 
-        self.latest_randao_mixes[(self.slot % spec.latest_randao_mixes_length) as usize] = new_mix;
+        self.latest_randao_mixes[(self.slot % spec.latest_randao_mixes_length).as_usize()] =
+            new_mix;
 
         /*
          * Eth1 data
@@ -593,7 +577,7 @@ impl BeaconState {
         Ok(())
     }
 
-    pub fn get_shuffling(&self, seed: Hash256, slot: u64, spec: &ChainSpec) -> Vec<Vec<usize>> {
+    pub fn get_shuffling(&self, seed: Hash256, slot: Slot, spec: &ChainSpec) -> Vec<Vec<usize>> {
         let slot = slot - (slot % spec.epoch_length);
 
         let active_validator_indices = get_active_validator_indices(&self.validator_registry, slot);
@@ -602,7 +586,7 @@ impl BeaconState {
             self.get_committee_count_per_slot(active_validator_indices.len(), spec);
 
         // TODO: check that Hash256 matches 'int_to_bytes32'.
-        let seed = seed ^ Hash256::from(slot);
+        let seed = seed ^ Hash256::from(slot.as_u64());
         let shuffled_active_validator_indices =
             shuffle(&seed, active_validator_indices).expect("Max validator count exceed!");
 
@@ -616,7 +600,7 @@ impl BeaconState {
     /// If the state does not contain an index for a beacon proposer at the requested `slot`, then `None` is returned.
     pub fn get_beacon_proposer_index(
         &self,
-        slot: u64,
+        slot: Slot,
         spec: &ChainSpec,
     ) -> Result<usize, CommitteesError> {
         let committees = self.get_crosslink_committees_at_slot(slot, spec)?;
@@ -624,7 +608,7 @@ impl BeaconState {
             .first()
             .ok_or(CommitteesError::InsufficientNumberOfValidators)
             .and_then(|(first_committee, _)| {
-                let index = (slot as usize)
+                let index = (slot.as_usize())
                     .checked_rem(first_committee.len())
                     .ok_or(CommitteesError::InsufficientNumberOfValidators)?;
                 // NOTE: next index will not panic as we have already returned if this is the case
@@ -653,7 +637,10 @@ impl BeaconState {
         let current_epoch_attestations: Vec<&PendingAttestation> = self
             .latest_attestations
             .par_iter()
-            .filter(|a| a.data.slot / spec.epoch_length == self.current_epoch(spec))
+            .filter(|a| {
+                (a.data.slot / spec.epoch_length).epoch(spec.epoch_length)
+                    == self.current_epoch(spec)
+            })
             .collect();
 
         debug!(
@@ -708,7 +695,8 @@ impl BeaconState {
             .par_iter()
             .filter(|a| {
                 //TODO: ensure these saturating subs are correct.
-                a.data.slot / spec.epoch_length == self.previous_epoch(spec)
+                (a.data.slot / spec.epoch_length).epoch(spec.epoch_length)
+                    == self.previous_epoch(spec)
             })
             .collect();
 
@@ -864,7 +852,7 @@ impl BeaconState {
             HashMap::new();
 
         // for slot in self.slot.saturating_sub(2 * spec.epoch_length)..self.slot {
-        for slot in self.get_previous_epoch_boundaries(spec) {
+        for slot in self.previous_epoch(spec).slot_iter(spec.epoch_length) {
             let crosslink_committees_at_slot = self.get_crosslink_committees_at_slot(slot, spec)?;
 
             for (crosslink_committee, shard) in crosslink_committees_at_slot {
@@ -909,7 +897,7 @@ impl BeaconState {
          * Justification and finalization
          */
         let epochs_since_finality =
-            self.slot.saturating_sub(self.finalized_slot) / spec.epoch_length;
+            (self.slot.saturating_sub(self.finalized_slot) / spec.epoch_length).as_u64();
 
         // TODO: fix this extra map
         let previous_epoch_justified_attester_indices_hashset: HashSet<usize> =
@@ -1028,7 +1016,7 @@ impl BeaconState {
         /*
          * Crosslinks
          */
-        for slot in self.get_previous_epoch_boundaries(spec) {
+        for slot in self.previous_epoch(spec).slot_iter(spec.epoch_length) {
             let crosslink_committees_at_slot = self.get_crosslink_committees_at_slot(slot, spec)?;
 
             for (_crosslink_committee, shard) in crosslink_committees_at_slot {
@@ -1097,8 +1085,7 @@ impl BeaconState {
                 + self.get_current_epoch_committee_count_per_slot(spec) as u64 * spec.epoch_length)
                 % spec.shard_count;
             self.current_epoch_seed = self.get_randao_mix(
-                self.current_epoch_calculation_slot
-                    .saturating_sub(spec.seed_lookahead),
+                self.current_epoch_calculation_slot - spec.seed_lookahead,
                 spec,
             );
         } else {
@@ -1107,8 +1094,7 @@ impl BeaconState {
             if epochs_since_last_registry_change.is_power_of_two() {
                 self.current_epoch_calculation_slot = self.slot;
                 self.current_epoch_seed = self.get_randao_mix(
-                    self.current_epoch_calculation_slot
-                        .saturating_sub(spec.seed_lookahead),
+                    self.current_epoch_calculation_slot - spec.seed_lookahead,
                     spec,
                 );
             }
@@ -1117,13 +1103,16 @@ impl BeaconState {
         self.process_penalties_and_exits(spec);
 
         let e = self.slot / spec.epoch_length;
-        self.latest_penalized_balances[((e + 1) % spec.latest_penalized_exit_length) as usize] =
-            self.latest_penalized_balances[(e % spec.latest_penalized_exit_length) as usize];
+        self.latest_penalized_balances[((e + 1) % spec.latest_penalized_exit_length).as_usize()] =
+            self.latest_penalized_balances[(e % spec.latest_penalized_exit_length).as_usize()];
 
         self.latest_attestations = self
             .latest_attestations
             .iter()
-            .filter(|a| a.data.slot / spec.epoch_length >= self.current_epoch(spec))
+            .filter(|a| {
+                (a.data.slot / spec.epoch_length).epoch(spec.epoch_length)
+                    >= self.current_epoch(spec)
+            })
             .cloned()
             .collect();
 
@@ -1146,8 +1135,8 @@ impl BeaconState {
             {
                 let e = (self.slot / spec.epoch_length) % spec.latest_penalized_exit_length;
                 let total_at_start = self.latest_penalized_balances
-                    [((e + 1) % spec.latest_penalized_exit_length) as usize];
-                let total_at_end = self.latest_penalized_balances[e as usize];
+                    [((e + 1) % spec.latest_penalized_exit_length).as_usize()];
+                let total_at_end = self.latest_penalized_balances[e.as_usize()];
                 let total_penalities = total_at_end.saturating_sub(total_at_start);
                 let penalty = self.get_effective_balance(index, spec)
                     * std::cmp::min(total_penalities * 3, total_balance)
@@ -1187,10 +1176,10 @@ impl BeaconState {
         self.validator_registry[index].status_flags = Some(StatusFlags::Withdrawable);
     }
 
-    fn get_randao_mix(&mut self, slot: u64, spec: &ChainSpec) -> Hash256 {
+    fn get_randao_mix(&mut self, slot: Slot, spec: &ChainSpec) -> Hash256 {
         assert!(self.slot < slot + spec.latest_randao_mixes_length);
         assert!(slot <= self.slot);
-        self.latest_randao_mixes[(slot & spec.latest_randao_mixes_length) as usize]
+        self.latest_randao_mixes[(slot % spec.latest_randao_mixes_length).as_usize()]
     }
 
     fn update_validator_registry(&mut self, spec: &ChainSpec) {
@@ -1260,7 +1249,7 @@ impl BeaconState {
         }
     }
 
-    fn entry_exit_effect_slot(&self, slot: u64, spec: &ChainSpec) -> u64 {
+    fn entry_exit_effect_slot(&self, slot: Slot, spec: &ChainSpec) -> Slot {
         (slot - slot % spec.epoch_length) + spec.epoch_length + spec.entry_exit_delay
     }
 
@@ -1288,9 +1277,7 @@ impl BeaconState {
     ) -> Result<u64, InclusionError> {
         let attestation =
             self.earliest_included_attestation(attestations, validator_index, spec)?;
-        Ok(attestation
-            .slot_included
-            .saturating_sub(attestation.data.slot))
+        Ok((attestation.slot_included - attestation.data.slot).as_u64())
     }
 
     fn inclusion_slot(
@@ -1298,7 +1285,7 @@ impl BeaconState {
         attestations: &[&PendingAttestation],
         validator_index: usize,
         spec: &ChainSpec,
-    ) -> Result<u64, InclusionError> {
+    ) -> Result<Slot, InclusionError> {
         let attestation =
             self.earliest_included_attestation(attestations, validator_index, spec)?;
         Ok(attestation.slot_included)
@@ -1350,10 +1337,10 @@ impl BeaconState {
         std::cmp::min(self.validator_balances[validator_index], spec.max_deposit)
     }
 
-    pub fn get_block_root(&self, slot: u64, spec: &ChainSpec) -> Option<&Hash256> {
+    pub fn get_block_root(&self, slot: Slot, spec: &ChainSpec) -> Option<&Hash256> {
         if self.slot <= slot + spec.latest_block_roots_length && slot <= self.slot {
             self.latest_block_roots
-                .get((slot % spec.latest_block_roots_length) as usize)
+                .get((slot % spec.latest_block_roots_length).as_usize())
         } else {
             None
         }
@@ -1589,7 +1576,7 @@ fn penalize_validator(_state: &BeaconState, _proposer_index: usize) {
     // TODO: stubbed out.
 }
 
-fn get_domain(_fork: &Fork, _slot: u64, _domain_type: u64) -> u64 {
+fn get_domain(_fork: &Fork, _slot: Slot, _domain_type: u64) -> u64 {
     // TODO: stubbed out.
     0
 }
diff --git a/eth2/types/src/readers/block_reader.rs b/eth2/types/src/readers/block_reader.rs
index d87bc5caf..bcb2d0e63 100644
--- a/eth2/types/src/readers/block_reader.rs
+++ b/eth2/types/src/readers/block_reader.rs
@@ -1,5 +1,4 @@
-use super::state_reader::BeaconStateReader;
-use crate::{BeaconBlock, Hash256};
+use crate::{BeaconBlock, Hash256, Slot};
 use std::fmt::Debug;
 
 /// The `BeaconBlockReader` provides interfaces for reading a subset of fields of a `BeaconBlock`.
@@ -11,7 +10,7 @@ use std::fmt::Debug;
 /// Note: presently, direct SSZ reading has not been implemented so this trait is being used for
 /// "future proofing".
 pub trait BeaconBlockReader: Debug + PartialEq {
-    fn slot(&self) -> u64;
+    fn slot(&self) -> Slot;
     fn parent_root(&self) -> Hash256;
     fn state_root(&self) -> Hash256;
     fn canonical_root(&self) -> Hash256;
@@ -19,7 +18,7 @@ pub trait BeaconBlockReader: Debug + PartialEq {
 }
 
 impl BeaconBlockReader for BeaconBlock {
-    fn slot(&self) -> u64 {
+    fn slot(&self) -> Slot {
         self.slot
     }
 
diff --git a/eth2/types/src/readers/state_reader.rs b/eth2/types/src/readers/state_reader.rs
index 47266bd36..92a870855 100644
--- a/eth2/types/src/readers/state_reader.rs
+++ b/eth2/types/src/readers/state_reader.rs
@@ -1,4 +1,4 @@
-use crate::{BeaconState, Hash256};
+use crate::{BeaconState, Hash256, Slot};
 use std::fmt::Debug;
 
 /// The `BeaconStateReader` provides interfaces for reading a subset of fields of a `BeaconState`.
@@ -10,13 +10,13 @@ use std::fmt::Debug;
 /// Note: presently, direct SSZ reading has not been implemented so this trait is being used for
 /// "future proofing".
 pub trait BeaconStateReader: Debug + PartialEq {
-    fn slot(&self) -> u64;
+    fn slot(&self) -> Slot;
     fn canonical_root(&self) -> Hash256;
     fn into_beacon_state(self) -> Option<BeaconState>;
 }
 
 impl BeaconStateReader for BeaconState {
-    fn slot(&self) -> u64 {
+    fn slot(&self) -> Slot {
         self.slot
     }
 
diff --git a/eth2/types/src/slot_epoch.rs b/eth2/types/src/slot_epoch.rs
index 4996722e4..40b0c5e95 100644
--- a/eth2/types/src/slot_epoch.rs
+++ b/eth2/types/src/slot_epoch.rs
@@ -10,13 +10,13 @@
 /// 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 crate::ChainSpec;
 use rand::RngCore;
 use serde_derive::Serialize;
 use ssz::{hash, Decodable, DecodeError, Encodable, SszStream, TreeHash};
 use std::cmp::{Ord, Ordering};
 use std::fmt;
-use std::ops::{Add, AddAssign, Rem, Sub, SubAssign};
+use std::iter::Iterator;
+use std::ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Rem, Sub, SubAssign};
 
 macro_rules! impl_from_into_u64 {
     ($main: ident) => {
@@ -88,7 +88,7 @@ macro_rules! impl_math_between {
 
         impl AddAssign<$other> for $main {
             fn add_assign(&mut self, other: $other) {
-                self.0.saturating_add(other.into());
+                self.0 = self.0.saturating_add(other.into());
             }
         }
 
@@ -102,7 +102,45 @@ macro_rules! impl_math_between {
 
         impl SubAssign<$other> for $main {
             fn sub_assign(&mut self, other: $other) {
-                self.0.saturating_sub(other.into());
+                self.0 = self.0.saturating_sub(other.into());
+            }
+        }
+
+        impl Mul<$other> for $main {
+            type Output = $main;
+
+            fn mul(self, rhs: $other) -> $main {
+                let rhs: u64 = rhs.into();
+                $main::from(self.0.saturating_mul(rhs))
+            }
+        }
+
+        impl MulAssign<$other> for $main {
+            fn mul_assign(&mut self, rhs: $other) {
+                let rhs: u64 = rhs.into();
+                self.0 = self.0.saturating_mul(rhs)
+            }
+        }
+
+        impl Div<$other> for $main {
+            type Output = $main;
+
+            fn div(self, rhs: $other) -> $main {
+                let rhs: u64 = rhs.into();
+                if rhs == 0 {
+                    panic!("Cannot divide by zero-valued Slot/Epoch")
+                }
+                $main::from(self.0 / rhs)
+            }
+        }
+
+        impl DivAssign<$other> for $main {
+            fn div_assign(&mut self, rhs: $other) {
+                let rhs: u64 = rhs.into();
+                if rhs == 0 {
+                    panic!("Cannot divide by zero-valued Slot/Epoch")
+                }
+                self.0 = self.0 / rhs
             }
         }
 
@@ -117,6 +155,27 @@ macro_rules! impl_math_between {
     };
 }
 
+macro_rules! impl_math {
+    ($type: ident) => {
+        impl $type {
+            pub fn saturating_sub<T: Into<$type>>(&self, other: T) -> $type {
+                *self - other.into()
+            }
+
+            pub fn is_power_of_two(&self) -> bool {
+                self.0.is_power_of_two()
+            }
+        }
+
+        impl Ord for $type {
+            fn cmp(&self, other: &$type) -> Ordering {
+                let other: u64 = (*other).into();
+                self.0.cmp(&other)
+            }
+        }
+    };
+}
+
 macro_rules! impl_display {
     ($type: ident) => {
         impl fmt::Display for $type {
@@ -169,6 +228,7 @@ impl_from_into_u64!(Slot);
 impl_from_into_usize!(Slot);
 impl_math_between!(Slot, Slot);
 impl_math_between!(Slot, u64);
+impl_math!(Slot);
 impl_display!(Slot);
 impl_ssz!(Slot);
 
@@ -176,33 +236,59 @@ impl_from_into_u64!(Epoch);
 impl_from_into_usize!(Epoch);
 impl_math_between!(Epoch, Epoch);
 impl_math_between!(Epoch, u64);
+impl_math!(Epoch);
 impl_display!(Epoch);
 impl_ssz!(Epoch);
 
 impl Slot {
-    pub fn epoch(&self, spec: &ChainSpec) -> Epoch {
-        Epoch::from(self.0 / spec.epoch_length)
+    pub fn epoch(&self, epoch_length: u64) -> Epoch {
+        Epoch::from(self.0 / epoch_length)
+    }
+
+    pub fn max_value() -> Slot {
+        Slot(u64::max_value())
     }
 }
 
 impl Epoch {
-    pub fn start_slot(&self, spec: &ChainSpec) -> Slot {
-        Slot::from(self.0.saturating_mul(spec.epoch_length))
+    pub fn start_slot(&self, epoch_length: u64) -> Slot {
+        Slot::from(self.0.saturating_mul(epoch_length))
     }
 
-    pub fn end_slot(&self, spec: &ChainSpec) -> Slot {
+    pub fn end_slot(&self, epoch_length: u64) -> Slot {
         Slot::from(
             self.0
                 .saturating_add(1)
-                .saturating_mul(spec.epoch_length)
+                .saturating_mul(epoch_length)
                 .saturating_sub(1),
         )
     }
 
-    pub fn slots(&self, spec: &ChainSpec) -> Vec<Slot> {
-        (self.start_slot(spec).as_u64()..self.end_slot(spec).as_u64())
-            .into_iter()
-            .map(|i| Slot::from(i))
-            .collect()
+    pub fn slot_iter(&self, epoch_length: u64) -> SlotIter {
+        SlotIter {
+            current: self.start_slot(epoch_length),
+            epoch: self,
+            epoch_length,
+        }
+    }
+}
+
+pub struct SlotIter<'a> {
+    current: Slot,
+    epoch: &'a Epoch,
+    epoch_length: u64,
+}
+
+impl<'a> Iterator for SlotIter<'a> {
+    type Item = Slot;
+
+    fn next(&mut self) -> Option<Slot> {
+        if self.current == self.epoch.end_slot(self.epoch_length) {
+            None
+        } else {
+            let previous = self.current;
+            self.current += 1;
+            Some(previous)
+        }
     }
 }
diff --git a/eth2/types/src/spec/foundation.rs b/eth2/types/src/spec/foundation.rs
index f24a55b2a..46f16ecd9 100644
--- a/eth2/types/src/spec/foundation.rs
+++ b/eth2/types/src/spec/foundation.rs
@@ -1,7 +1,7 @@
 use super::ChainSpec;
 use bls::{Keypair, PublicKey, SecretKey, Signature};
 
-use crate::{Address, Eth1Data, Hash256, Validator};
+use crate::{Address, Eth1Data, Hash256, Slot, Validator};
 
 /// The size of a validators deposit in GWei.
 pub const DEPOSIT_GWEI: u64 = 32_000_000_000;
@@ -37,9 +37,9 @@ impl ChainSpec {
              * Initial Values
              */
             genesis_fork_version: 0,
-            genesis_slot: 0,
+            genesis_slot: Slot::from(0_u64),
             genesis_start_shard: 0,
-            far_future_slot: u64::max_value(),
+            far_future_slot: Slot::from(u64::max_value()),
             zero_hash: Hash256::zero(),
             empty_signature: Signature::empty_signature(),
             bls_withdrawal_prefix_byte: 0x00,
@@ -109,15 +109,15 @@ fn initial_validators_for_testing() -> Vec<Validator> {
         let validator = Validator {
             pubkey: keypair.pk.clone(),
             withdrawal_credentials: Hash256::zero(),
-            proposer_slots: 0,
-            activation_slot: u64::max_value(),
-            exit_slot: u64::max_value(),
-            withdrawal_slot: u64::max_value(),
-            penalized_slot: u64::max_value(),
+            proposer_slots: Slot::from(0_u64),
+            activation_slot: Slot::max_value(),
+            exit_slot: Slot::max_value(),
+            withdrawal_slot: Slot::max_value(),
+            penalized_slot: Slot::max_value(),
             exit_count: 0,
             status_flags: None,
-            latest_custody_reseed_slot: 0,
-            penultimate_custody_reseed_slot: 0,
+            latest_custody_reseed_slot: Slot::from(0_u64),
+            penultimate_custody_reseed_slot: Slot::from(0_u64),
         };
         initial_validators.push(validator);
     }
diff --git a/eth2/types/src/spec/mod.rs b/eth2/types/src/spec/mod.rs
index 077b6bef5..5007a26b0 100644
--- a/eth2/types/src/spec/mod.rs
+++ b/eth2/types/src/spec/mod.rs
@@ -1,6 +1,6 @@
 mod foundation;
 
-use crate::{Address, Eth1Data, Hash256, Validator};
+use crate::{Address, Eth1Data, Hash256, Slot, Validator};
 use bls::Signature;
 
 #[derive(PartialEq, Debug, Clone)]
@@ -29,9 +29,9 @@ pub struct ChainSpec {
      * Initial Values
      */
     pub genesis_fork_version: u64,
-    pub genesis_slot: u64,
+    pub genesis_slot: Slot,
     pub genesis_start_shard: u64,
-    pub far_future_slot: u64,
+    pub far_future_slot: Slot,
     pub zero_hash: Hash256,
     pub empty_signature: Signature,
     pub bls_withdrawal_prefix_byte: u8,
diff --git a/eth2/types/src/validator_registry.rs b/eth2/types/src/validator_registry.rs
index 509ab7c7b..236d512b6 100644
--- a/eth2/types/src/validator_registry.rs
+++ b/eth2/types/src/validator_registry.rs
@@ -1,9 +1,10 @@
 /// Contains logic to manipulate a `&[Validator]`.
 /// For now, we avoid defining a newtype and just have flat functions here.
 use super::validator::*;
+use crate::Slot;
 
 /// Given an indexed sequence of `validators`, return the indices corresponding to validators that are active at `slot`.
-pub fn get_active_validator_indices(validators: &[Validator], slot: u64) -> Vec<usize> {
+pub fn get_active_validator_indices(validators: &[Validator], slot: Slot) -> Vec<usize> {
     validators
         .iter()
         .enumerate()
@@ -27,7 +28,7 @@ mod tests {
         let mut rng = XorShiftRng::from_seed([42; 16]);
 
         let validators = vec![];
-        let some_slot = u64::random_for_test(&mut rng);
+        let some_slot = Slot::random_for_test(&mut rng);
         let indices = get_active_validator_indices(&validators, some_slot);
         assert_eq!(indices, vec![]);
     }
@@ -41,7 +42,7 @@ mod tests {
             validators.push(Validator::default())
         }
 
-        let some_slot = u64::random_for_test(&mut rng);
+        let some_slot = Slot::random_for_test(&mut rng);
         let indices = get_active_validator_indices(&validators, some_slot);
         assert_eq!(indices, vec![]);
     }
@@ -50,7 +51,7 @@ mod tests {
     fn can_get_all_active_validator_indices() {
         let mut rng = XorShiftRng::from_seed([42; 16]);
         let count_validators = 10;
-        let some_slot = u64::random_for_test(&mut rng);
+        let some_slot = Slot::random_for_test(&mut rng);
 
         let mut validators = (0..count_validators)
             .into_iter()
@@ -60,8 +61,8 @@ mod tests {
                 let activation_offset = u64::random_for_test(&mut rng);
                 let exit_offset = u64::random_for_test(&mut rng);
 
-                validator.activation_slot = some_slot.checked_sub(activation_offset).unwrap_or(0);
-                validator.exit_slot = some_slot.checked_add(exit_offset).unwrap_or(std::u64::MAX);
+                validator.activation_slot = some_slot - activation_offset;
+                validator.exit_slot = some_slot + exit_offset;
 
                 validator
             })
@@ -81,13 +82,13 @@ mod tests {
 
     fn set_validators_to_default_entry_exit(validators: &mut [Validator]) {
         for validator in validators.iter_mut() {
-            validator.activation_slot = std::u64::MAX;
-            validator.exit_slot = std::u64::MAX;
+            validator.activation_slot = Slot::max_value();
+            validator.exit_slot = Slot::max_value();
         }
     }
 
     // sets all `validators` to be active as of some slot prior to `slot`. returns the activation slot.
-    fn set_validators_to_activated(validators: &mut [Validator], slot: u64) -> u64 {
+    fn set_validators_to_activated(validators: &mut [Validator], slot: Slot) -> Slot {
         let activation_slot = slot - 10;
         for validator in validators.iter_mut() {
             validator.activation_slot = activation_slot;
@@ -96,7 +97,7 @@ mod tests {
     }
 
     // sets all `validators` to be exited as of some slot before `slot`.
-    fn set_validators_to_exited(validators: &mut [Validator], slot: u64, activation_slot: u64) {
+    fn set_validators_to_exited(validators: &mut [Validator], slot: Slot, activation_slot: Slot) {
         assert!(activation_slot < slot);
         let mut exit_slot = activation_slot + 10;
         while exit_slot >= slot {
@@ -114,18 +115,18 @@ mod tests {
         let mut rng = XorShiftRng::from_seed([42; 16]);
         const COUNT_PARTITIONS: usize = 3;
         const COUNT_VALIDATORS: usize = 3 * COUNT_PARTITIONS;
-        let some_slot: u64 = u64::random_for_test(&mut rng);
+        let some_slot: Slot = Slot::random_for_test(&mut rng);
 
         let mut validators = (0..COUNT_VALIDATORS)
             .into_iter()
             .map(|_| {
                 let mut validator = Validator::default();
 
-                let activation_offset = u64::random_for_test(&mut rng);
-                let exit_offset = u64::random_for_test(&mut rng);
+                let activation_offset = Slot::random_for_test(&mut rng);
+                let exit_offset = Slot::random_for_test(&mut rng);
 
-                validator.activation_slot = some_slot.checked_sub(activation_offset).unwrap_or(0);
-                validator.exit_slot = some_slot.checked_add(exit_offset).unwrap_or(std::u64::MAX);
+                validator.activation_slot = some_slot - activation_offset;
+                validator.exit_slot = some_slot + exit_offset;
 
                 validator
             })