diff --git a/eth2/state_processing/src/block_processable.rs b/eth2/state_processing/src/block_processable.rs
index ee8bc7c33..6f4d7268e 100644
--- a/eth2/state_processing/src/block_processable.rs
+++ b/eth2/state_processing/src/block_processable.rs
@@ -5,7 +5,11 @@ use int_to_bytes::int_to_bytes32;
 use log::{debug, trace};
 use ssz::{ssz_encode, TreeHash};
 use types::*;
+use validate_attestation::validate_attestations;
 
+pub use validate_attestation::{validate_attestation, validate_attestation_without_signature};
+
+mod validate_attestation;
 mod verify_slashable_attestation;
 
 const PHASE_0_CUSTODY_BIT: bool = false;
@@ -41,7 +45,6 @@ pub enum Error {
     BeaconStateError(BeaconStateError),
     SlotProcessingError(SlotProcessingError),
 }
-
 #[derive(Debug, PartialEq)]
 pub enum AttestationValidationError {
     IncludedTooEarly,
@@ -221,19 +224,10 @@ fn per_block_processing_signature_optional(
         verify_slashable_attestation(&mut state, &attester_slashing, spec)?;
     }
 
-    /*
-     * Attestations
-     */
-    ensure!(
-        block.body.attestations.len() as u64 <= spec.max_attestations,
-        Error::MaxAttestationsExceeded
-    );
-
-    debug!("Verifying {} attestations.", block.body.attestations.len());
+    validate_attestations(&mut state, &block, spec);
 
+    // Convert each attestation into a `PendingAttestation` and insert into the state.
     for attestation in &block.body.attestations {
-        validate_attestation(&state, attestation, spec)?;
-
         let pending_attestation = PendingAttestation {
             data: attestation.data.clone(),
             aggregation_bitfield: attestation.aggregation_bitfield.clone(),
@@ -293,7 +287,7 @@ fn per_block_processing_signature_optional(
         );
         ensure!(state.current_epoch(spec) >= exit.epoch, Error::BadExit);
         let exit_message = {
-            let exit_struct = Exit {
+            let exit_struct = VoluntaryExit {
                 epoch: exit.epoch,
                 validator_index: exit.validator_index,
                 signature: spec.empty_signature.clone(),
@@ -317,110 +311,6 @@ fn per_block_processing_signature_optional(
     Ok(())
 }
 
-pub fn validate_attestation(
-    state: &BeaconState,
-    attestation: &Attestation,
-    spec: &ChainSpec,
-) -> Result<(), AttestationValidationError> {
-    validate_attestation_signature_optional(state, attestation, spec, true)
-}
-
-pub fn validate_attestation_without_signature(
-    state: &BeaconState,
-    attestation: &Attestation,
-    spec: &ChainSpec,
-) -> Result<(), AttestationValidationError> {
-    validate_attestation_signature_optional(state, attestation, spec, false)
-}
-
-fn validate_attestation_signature_optional(
-    state: &BeaconState,
-    attestation: &Attestation,
-    spec: &ChainSpec,
-    verify_signature: bool,
-) -> Result<(), AttestationValidationError> {
-    trace!(
-        "validate_attestation_signature_optional: attestation epoch: {}",
-        attestation.data.slot.epoch(spec.slots_per_epoch)
-    );
-    ensure!(
-        attestation.data.slot + spec.min_attestation_inclusion_delay <= state.slot,
-        AttestationValidationError::IncludedTooEarly
-    );
-    ensure!(
-        attestation.data.slot + spec.slots_per_epoch >= state.slot,
-        AttestationValidationError::IncludedTooLate
-    );
-    if attestation.data.slot >= state.current_epoch_start_slot(spec) {
-        ensure!(
-            attestation.data.justified_epoch == state.justified_epoch,
-            AttestationValidationError::WrongJustifiedSlot
-        );
-    } else {
-        ensure!(
-            attestation.data.justified_epoch == state.previous_justified_epoch,
-            AttestationValidationError::WrongJustifiedSlot
-        );
-    }
-    ensure!(
-        attestation.data.justified_block_root
-            == *state
-                .get_block_root(
-                    attestation
-                        .data
-                        .justified_epoch
-                        .start_slot(spec.slots_per_epoch),
-                    &spec
-                )
-                .ok_or(AttestationValidationError::NoBlockRoot)?,
-        AttestationValidationError::WrongJustifiedRoot
-    );
-    let potential_crosslink = Crosslink {
-        shard_block_root: attestation.data.shard_block_root,
-        epoch: attestation.data.slot.epoch(spec.slots_per_epoch),
-    };
-    ensure!(
-        (attestation.data.latest_crosslink
-            == state.latest_crosslinks[attestation.data.shard as usize])
-            | (attestation.data.latest_crosslink == potential_crosslink),
-        AttestationValidationError::BadLatestCrosslinkRoot
-    );
-    if verify_signature {
-        let participants = state.get_attestation_participants(
-            &attestation.data,
-            &attestation.aggregation_bitfield,
-            spec,
-        )?;
-        trace!(
-            "slot: {}, shard: {}, participants: {:?}",
-            attestation.data.slot,
-            attestation.data.shard,
-            participants
-        );
-        let mut group_public_key = AggregatePublicKey::new();
-        for participant in participants {
-            group_public_key.add(&state.validator_registry[participant as usize].pubkey)
-        }
-        ensure!(
-            attestation.verify_signature(
-                &group_public_key,
-                PHASE_0_CUSTODY_BIT,
-                get_domain(
-                    &state.fork,
-                    attestation.data.slot.epoch(spec.slots_per_epoch),
-                    spec.domain_attestation,
-                )
-            ),
-            AttestationValidationError::BadSignature
-        );
-    }
-    ensure!(
-        attestation.data.shard_block_root == spec.zero_hash,
-        AttestationValidationError::ShardBlockRootNotZero
-    );
-    Ok(())
-}
-
 fn get_domain(fork: &Fork, epoch: Epoch, domain_type: u64) -> u64 {
     fork.get_domain(epoch, domain_type)
 }
diff --git a/eth2/state_processing/src/block_processable/validate_attestation.rs b/eth2/state_processing/src/block_processable/validate_attestation.rs
new file mode 100644
index 000000000..1b422711c
--- /dev/null
+++ b/eth2/state_processing/src/block_processable/validate_attestation.rs
@@ -0,0 +1,238 @@
+use crate::errors::{AttestationInvalid as Invalid, AttestationValidationError as Error};
+use ssz::TreeHash;
+use types::beacon_state::helpers::*;
+use types::*;
+
+/// Validate the attestations in some block, converting each into a `PendingAttestation` which is
+/// then added to `state.latest_attestations`.
+///
+/// Spec v0.4.0
+pub fn validate_attestations(
+    state: &BeaconState,
+    block: &BeaconBlock,
+    spec: &ChainSpec,
+) -> Result<(), Error> {
+    ensure!(
+        block.body.attestations.len() as u64 <= spec.max_attestations,
+        MaxAttestationsExceeded
+    );
+
+    for attestation in &block.body.attestations {
+        validate_attestation(&state, attestation, spec)?;
+    }
+
+    Ok(())
+}
+
+/// Validate an attestation, checking the aggregate signature.
+///
+/// Spec v0.4.0
+pub fn validate_attestation(
+    state: &BeaconState,
+    attestation: &Attestation,
+    spec: &ChainSpec,
+) -> Result<(), Error> {
+    validate_attestation_signature_optional(state, attestation, spec, true)
+}
+
+/// Validate an attestation, without checking the aggregate signature.
+///
+/// Spec v0.4.0
+pub fn validate_attestation_without_signature(
+    state: &BeaconState,
+    attestation: &Attestation,
+    spec: &ChainSpec,
+) -> Result<(), Error> {
+    validate_attestation_signature_optional(state, attestation, spec, false)
+}
+
+/// Validate an attestation, optionally checking the aggregate signature.
+///
+/// Spec v0.2.0
+fn validate_attestation_signature_optional(
+    state: &BeaconState,
+    attestation: &Attestation,
+    spec: &ChainSpec,
+    verify_signature: bool,
+) -> Result<(), Error> {
+    // Verify that `attestation.data.slot >= GENESIS_SLOT`.
+    ensure!(attestation.data.slot >= spec.genesis_slot, PreGenesis);
+
+    // Verify that `attestation.data.slot + MIN_ATTESTATION_INCLUSION_DELAY <= state.slot`.
+    ensure!(
+        attestation.data.slot + spec.min_attestation_inclusion_delay <= state.slot,
+        IncludedTooEarly
+    );
+
+    // Verify that `state.slot < attestation.data.slot + SLOTS_PER_EPOCH`.
+    ensure!(
+        state.slot < attestation.data.slot + spec.slots_per_epoch,
+        IncludedTooLate
+    );
+
+    // Verify that `attestation.data.justified_epoch` is equal to `state.justified_epoch` if
+    // `slot_to_epoch(attestation.data.slot + 1) >= get_current_epoch(state) else
+    // state.previous_justified_epoch`.
+    if (attestation.data.slot + 1).epoch(spec.slots_per_epoch) >= state.current_epoch(spec) {
+        ensure!(
+            attestation.data.justified_epoch == state.justified_epoch,
+            WrongJustifiedSlot
+        );
+    } else {
+        ensure!(
+            attestation.data.justified_epoch == state.previous_justified_epoch,
+            WrongJustifiedSlot
+        );
+    }
+
+    // Verify that `attestation.data.justified_block_root` is equal to `get_block_root(state,
+    // get_epoch_start_slot(attestation.data.justified_epoch))`.
+    ensure!(
+        attestation.data.justified_block_root
+            == *state
+                .get_block_root(
+                    attestation
+                        .data
+                        .justified_epoch
+                        .start_slot(spec.slots_per_epoch),
+                    &spec
+                )
+                .ok_or(BeaconStateError::InsufficientBlockRoots)?,
+        WrongJustifiedRoot
+    );
+
+    // Verify that either:
+    //
+    // (i)`state.latest_crosslinks[attestation.data.shard] == attestation.data.latest_crosslink`,
+    //
+    // (ii) `state.latest_crosslinks[attestation.data.shard] ==
+    // Crosslink(crosslink_data_root=attestation.data.crosslink_data_root,
+    // epoch=slot_to_epoch(attestation.data.slot))`.
+    let potential_crosslink = Crosslink {
+        crosslink_data_root: attestation.data.crosslink_data_root,
+        epoch: attestation.data.slot.epoch(spec.slots_per_epoch),
+    };
+    ensure!(
+        (attestation.data.latest_crosslink
+            == state.latest_crosslinks[attestation.data.shard as usize])
+            | (state.latest_crosslinks[attestation.data.shard as usize] == potential_crosslink),
+        BadLatestCrosslinkRoot
+    );
+
+    // Get the committee for this attestation
+    let (committee, _shard) = state
+        .get_crosslink_committees_at_slot(attestation.data.slot, spec)?
+        .iter()
+        .find(|(committee, shard)| *shard == attestation.data.shard)
+        .ok_or_else(|| Error::Invalid(Invalid::NoCommitteeForShard))?;
+
+    // Custody bitfield is all zeros (phase 0 requirement).
+    ensure!(
+        attestation.custody_bitfield.num_set_bits() == 0,
+        CustodyBitfieldHasSetBits
+    );
+    // Custody bitfield length is correct.
+    ensure!(
+        verify_bitfield_length(&attestation.aggregation_bitfield, committee.len()),
+        BadCustodyBitfieldLength
+    );
+    // Aggregation bitfield isn't empty.
+    ensure!(
+        attestation.aggregation_bitfield.num_set_bits() != 0,
+        AggregationBitfieldIsEmpty
+    );
+    // Aggregation bitfield length is correct.
+    ensure!(
+        verify_bitfield_length(&attestation.aggregation_bitfield, committee.len()),
+        BadAggregationBitfieldLength
+    );
+
+    if verify_signature {
+        ensure!(
+            verify_attestation_signature(
+                state,
+                committee,
+                &attestation.custody_bitfield,
+                &attestation.data,
+                &attestation.aggregate_signature,
+                spec
+            ),
+            BadSignature
+        );
+    }
+
+    // [TO BE REMOVED IN PHASE 1] Verify that `attestation.data.crosslink_data_root == ZERO_HASH`.
+    ensure!(
+        attestation.data.crosslink_data_root == spec.zero_hash,
+        ShardBlockRootNotZero
+    );
+
+    Ok(())
+}
+
+/// Verifies an aggregate signature for some given `AttestationData`, returning `true` if the
+/// `aggregate_signature` is valid.
+///
+/// Returns `false` if:
+///  - `aggregate_signature` was not signed correctly.
+///  - `custody_bitfield` does not have a bit for each index of `committee`.
+///  - A `validator_index` in `committee` is not in `state.validator_registry`.
+fn verify_attestation_signature(
+    state: &BeaconState,
+    committee: &[usize],
+    custody_bitfield: &Bitfield,
+    attestation_data: &AttestationData,
+    aggregate_signature: &AggregateSignature,
+    spec: &ChainSpec,
+) -> bool {
+    let mut aggregate_pubs = vec![AggregatePublicKey::new(); 2];
+    let mut message_exists = vec![false; 2];
+
+    for (i, v) in committee.iter().enumerate() {
+        let custody_bit = match custody_bitfield.get(i) {
+            Ok(bit) => bit,
+            // Invalidate signature if custody_bitfield.len() < committee
+            Err(_) => return false,
+        };
+
+        message_exists[custody_bit as usize] = true;
+
+        match state.validator_registry.get(*v as usize) {
+            Some(validator) => {
+                aggregate_pubs[custody_bit as usize].add(&validator.pubkey);
+            }
+            // Invalidate signature if validator index is unknown.
+            None => return false,
+        };
+    }
+
+    // Message when custody bitfield is `false`
+    let message_0 = AttestationDataAndCustodyBit {
+        data: attestation_data.clone(),
+        custody_bit: false,
+    }
+    .hash_tree_root();
+
+    // Message when custody bitfield is `true`
+    let message_1 = AttestationDataAndCustodyBit {
+        data: attestation_data.clone(),
+        custody_bit: true,
+    }
+    .hash_tree_root();
+
+    let mut messages = vec![];
+    let mut keys = vec![];
+
+    // If any validator signed a message with a `false` custody bit.
+    if message_exists[0] {
+        messages.push(&message_0[..]);
+        keys.push(&aggregate_pubs[0]);
+    }
+    // If any validator signed a message with a `true` custody bit.
+    if message_exists[1] {
+        messages.push(&message_1[..]);
+        keys.push(&aggregate_pubs[1]);
+    }
+
+    aggregate_signature.verify_multiple(&messages[..], spec.domain_attestation, &keys[..])
+}
diff --git a/eth2/state_processing/src/errors.rs b/eth2/state_processing/src/errors.rs
new file mode 100644
index 000000000..5eceb0be0
--- /dev/null
+++ b/eth2/state_processing/src/errors.rs
@@ -0,0 +1,30 @@
+use types::BeaconStateError;
+
+#[derive(Debug, PartialEq)]
+pub enum AttestationValidationError {
+    Invalid(AttestationInvalid),
+    ProcessingError(BeaconStateError),
+}
+
+#[derive(Debug, PartialEq)]
+pub enum AttestationInvalid {
+    PreGenesis,
+    IncludedTooEarly,
+    IncludedTooLate,
+    WrongJustifiedSlot,
+    WrongJustifiedRoot,
+    BadLatestCrosslinkRoot,
+    CustodyBitfieldHasSetBits,
+    AggregationBitfieldIsEmpty,
+    BadAggregationBitfieldLength,
+    BadCustodyBitfieldLength,
+    NoCommitteeForShard,
+    BadSignature,
+    ShardBlockRootNotZero,
+}
+
+impl From<BeaconStateError> for AttestationValidationError {
+    fn from(e: BeaconStateError) -> AttestationValidationError {
+        AttestationValidationError::ProcessingError(e)
+    }
+}
diff --git a/eth2/state_processing/src/lib.rs b/eth2/state_processing/src/lib.rs
index 18d1f7554..0a4343e00 100644
--- a/eth2/state_processing/src/lib.rs
+++ b/eth2/state_processing/src/lib.rs
@@ -1,10 +1,13 @@
+#[macro_use]
+mod macros;
 mod block_processable;
-mod epoch_processable;
-mod slot_processable;
+// mod epoch_processable;
+mod errors;
+// mod slot_processable;
 
 pub use block_processable::{
     validate_attestation, validate_attestation_without_signature, BlockProcessable,
     Error as BlockProcessingError,
 };
-pub use epoch_processable::{EpochProcessable, Error as EpochProcessingError};
-pub use slot_processable::{Error as SlotProcessingError, SlotProcessable};
+// pub use epoch_processable::{EpochProcessable, Error as EpochProcessingError};
+// pub use slot_processable::{Error as SlotProcessingError, SlotProcessable};
diff --git a/eth2/state_processing/src/macros.rs b/eth2/state_processing/src/macros.rs
new file mode 100644
index 000000000..6ccc54ba9
--- /dev/null
+++ b/eth2/state_processing/src/macros.rs
@@ -0,0 +1,8 @@
+#[macro_use]
+macro_rules! ensure {
+    ($condition: expr, $result: ident) => {
+        if !$condition {
+            return Err(Error::Invalid(Invalid::$result));
+        }
+    };
+}