diff --git a/beacon_node/beacon_chain/Cargo.toml b/beacon_node/beacon_chain/Cargo.toml
index 7e6dc209d..5c930403c 100644
--- a/beacon_node/beacon_chain/Cargo.toml
+++ b/beacon_node/beacon_chain/Cargo.toml
@@ -13,6 +13,7 @@ failure = "0.1"
 failure_derive = "0.1"
 genesis = { path = "../../eth2/genesis" }
 hashing = { path = "../../eth2/utils/hashing" }
+parking_lot = "0.7"
 log = "0.4"
 env_logger = "0.6"
 serde = "1.0"
diff --git a/beacon_node/beacon_chain/src/attestation_processing.rs b/beacon_node/beacon_chain/src/attestation_processing.rs
index ecbe355df..1b7e8ace4 100644
--- a/beacon_node/beacon_chain/src/attestation_processing.rs
+++ b/beacon_node/beacon_chain/src/attestation_processing.rs
@@ -19,15 +19,9 @@ where
         &self,
         free_attestation: FreeAttestation,
     ) -> Result<ProcessOutcome, Error> {
-        let present_slot = self
-            .present_slot()
-            .ok_or_else(|| Error::PresentSlotUnknown)?;
-        let state = self.state(present_slot)?;
-
         self.attestation_aggregator
             .write()
-            .expect("Aggregator unlock failed.")
-            .process_free_attestation(&state, &free_attestation, &self.spec)
+            .process_free_attestation(&self.state.read(), &free_attestation, &self.spec)
             .map_err(|e| e.into())
     }
 }
diff --git a/beacon_node/beacon_chain/src/attestation_production.rs b/beacon_node/beacon_chain/src/attestation_production.rs
index 9755f48af..febd2ff9a 100644
--- a/beacon_node/beacon_chain/src/attestation_production.rs
+++ b/beacon_node/beacon_chain/src/attestation_production.rs
@@ -13,30 +13,27 @@ where
     T: ClientDB,
     U: SlotClock,
 {
-    pub fn produce_attestation_data(
-        &self,
-        slot: u64,
-        shard: u64,
-    ) -> Result<AttestationData, Error> {
-        let present_slot = self
-            .present_slot()
-            .ok_or_else(|| Error::PresentSlotUnknown)?;
-        let state = self.state(present_slot).map_err(|_| Error::StateError)?;
-
+    pub fn produce_attestation_data(&self, shard: u64) -> Result<AttestationData, Error> {
         let justified_slot = self.justified_slot();
-
-        let justified_block_root = *state
+        let justified_block_root = self
+            .state
+            .read()
             .get_block_root(justified_slot, &self.spec)
-            .ok_or_else(|| Error::SlotTooOld)?;
+            .ok_or_else(|| Error::SlotTooOld)?
+            .clone();
 
-        let head_slot = self.head().beacon_block.slot;
-        let previous_epoch_start_slot = head_slot - (head_slot % self.spec.epoch_length);
-        let epoch_boundary_root = *state
-            .get_block_root(previous_epoch_start_slot, &self.spec)
-            .ok_or_else(|| Error::SlotTooOld)?;
+        let epoch_boundary_root = self
+            .state
+            .read()
+            .get_block_root(
+                self.state.read().current_epoch_start_slot(&self.spec),
+                &self.spec,
+            )
+            .ok_or_else(|| Error::SlotTooOld)?
+            .clone();
 
         Ok(AttestationData {
-            slot,
+            slot: self.state.read().slot,
             shard,
             beacon_block_root: self.head().beacon_block_root.clone(),
             epoch_boundary_root,
diff --git a/beacon_node/beacon_chain/src/attestation_targets.rs b/beacon_node/beacon_chain/src/attestation_targets.rs
index d968abcf2..62f0bbdfa 100644
--- a/beacon_node/beacon_chain/src/attestation_targets.rs
+++ b/beacon_node/beacon_chain/src/attestation_targets.rs
@@ -28,18 +28,12 @@ where
     U: SlotClock,
 {
     pub fn insert_latest_attestation_target(&self, validator_index: u64, block_root: Hash256) {
-        let mut targets = self
-            .latest_attestation_targets
-            .write()
-            .expect("CRITICAL: CanonicalHead poisioned.");
+        let mut targets = self.latest_attestation_targets.write();
         targets.insert(validator_index, block_root);
     }
 
     pub fn get_latest_attestation_target(&self, validator_index: u64) -> Option<Hash256> {
-        let targets = self
-            .latest_attestation_targets
-            .read()
-            .expect("CRITICAL: CanonicalHead poisioned.");
+        let targets = self.latest_attestation_targets.read();
 
         match targets.get(validator_index) {
             Some(hash) => Some(hash.clone()),
diff --git a/beacon_node/beacon_chain/src/block_processing.rs b/beacon_node/beacon_chain/src/block_processing.rs
index b4126ed9e..2c637031e 100644
--- a/beacon_node/beacon_chain/src/block_processing.rs
+++ b/beacon_node/beacon_chain/src/block_processing.rs
@@ -1,9 +1,9 @@
-use super::state_transition::Error as TransitionError;
 use super::{BeaconChain, ClientDB, DBError, SlotClock};
 use log::debug;
 use slot_clock::{SystemTimeSlotClockError, TestingSlotClockError};
 use ssz::{ssz_encode, Encodable};
 use types::{
+    beacon_state::{BlockProcessingError, SlotProcessingError},
     readers::{BeaconBlockReader, BeaconStateReader},
     Hash256,
 };
@@ -16,7 +16,6 @@ pub enum ValidBlock {
 #[derive(Debug, PartialEq)]
 pub enum InvalidBlock {
     FutureSlot,
-    StateTransitionFailed(TransitionError),
     StateRootMismatch,
 }
 
@@ -29,31 +28,16 @@ pub enum Outcome {
 #[derive(Debug, PartialEq)]
 pub enum Error {
     DBError(String),
-    SlotClockError(SystemTimeSlotClockError),
-
-    NotImplemented,
-    PresentSlotIsNone,
     UnableToDecodeBlock,
+    PresentSlotIsNone,
+    SlotClockError(SystemTimeSlotClockError),
     MissingParentState(Hash256),
     InvalidParentState(Hash256),
     MissingBeaconBlock(Hash256),
     InvalidBeaconBlock(Hash256),
     MissingParentBlock(Hash256),
-    NoBlockProducer,
-    StateSlotMismatch,
-    BadBlockSignature,
-    BadRandaoSignature,
-    MaxProposerSlashingsExceeded,
-    BadProposerSlashing,
-    MaxAttestationsExceeded,
-    BadAttestation,
-    NoBlockRoot,
-    MaxDepositsExceeded,
-    MaxExitsExceeded,
-    BadExit,
-    BadCustodyReseeds,
-    BadCustodyChallenges,
-    BadCustodyResponses,
+    SlotProcessingError(SlotProcessingError),
+    PerBlockProcessingError(BlockProcessingError),
 }
 
 impl<T, U> BeaconChain<T, U>
@@ -99,14 +83,13 @@ where
             .into_beacon_state()
             .ok_or(Error::InvalidParentState(parent_state_root))?;
 
-        let state = match self.state_transition(parent_state, &block) {
-            Ok(state) => state,
-            Err(error) => {
-                return Ok(Outcome::InvalidBlock(InvalidBlock::StateTransitionFailed(
-                    error,
-                )));
-            }
-        };
+        let mut state = parent_state;
+
+        for _ in state.slot..present_slot {
+            state.per_slot_processing(parent_block_root.clone(), &self.spec)?;
+        }
+
+        state.per_block_processing(&block, &self.spec)?;
 
         let state_root = state.canonical_root();
 
@@ -131,6 +114,7 @@ where
                 state.clone(),
                 state_root.clone(),
             );
+            *self.state.write() = state.clone();
         }
 
         // The block was sucessfully processed.
@@ -144,6 +128,18 @@ impl From<DBError> for Error {
     }
 }
 
+impl From<SlotProcessingError> for Error {
+    fn from(e: SlotProcessingError) -> Error {
+        Error::SlotProcessingError(e)
+    }
+}
+
+impl From<BlockProcessingError> for Error {
+    fn from(e: BlockProcessingError) -> Error {
+        Error::PerBlockProcessingError(e)
+    }
+}
+
 impl From<TestingSlotClockError> for Error {
     fn from(_: TestingSlotClockError) -> Error {
         unreachable!(); // Testing clock never throws an error.
diff --git a/beacon_node/beacon_chain/src/block_production.rs b/beacon_node/beacon_chain/src/block_production.rs
index 2e8665a77..6c1fa10c4 100644
--- a/beacon_node/beacon_chain/src/block_production.rs
+++ b/beacon_node/beacon_chain/src/block_production.rs
@@ -1,10 +1,9 @@
-use super::state_transition::Error as TransitionError;
 use super::{BeaconChain, ClientDB, DBError, SlotClock};
 use bls::Signature;
 use log::debug;
-use slot_clock::TestingSlotClockError;
+use slot_clock::{SystemTimeSlotClockError, TestingSlotClockError};
 use types::{
-    beacon_state::SlotProcessingError,
+    beacon_state::{BlockProcessingError, SlotProcessingError},
     readers::{BeaconBlockReader, BeaconStateReader},
     BeaconBlock, BeaconBlockBody, BeaconState, Eth1Data, Hash256,
 };
@@ -12,9 +11,10 @@ use types::{
 #[derive(Debug, PartialEq)]
 pub enum Error {
     DBError(String),
-    StateTransitionError(TransitionError),
     PresentSlotIsNone,
     SlotProcessingError(SlotProcessingError),
+    PerBlockProcessingError(BlockProcessingError),
+    SlotClockError(SystemTimeSlotClockError),
 }
 
 impl<T, U> BeaconChain<T, U>
@@ -29,43 +29,31 @@ where
     where
         Error: From<<U>::Error>,
     {
-        // TODO: allow producing a block from a previous (or future?) slot.
-        let present_slot = self
-            .slot_clock
-            .present_slot()
-            .map_err(|e| e.into())?
-            .ok_or(Error::PresentSlotIsNone)?;
+        debug!("Starting block production...");
 
-        debug!("Producing block for slot {}...", present_slot);
+        let mut state = self.state.read().clone();
 
-        let parent_root = self.head().beacon_block_root;
-        let parent_block_reader = self
-            .block_store
-            .get_reader(&parent_root)?
-            .ok_or_else(|| Error::DBError("Block not found.".to_string()))?;
-        let parent_state = self
-            .state_store
-            .get_reader(&parent_block_reader.state_root())?
-            .ok_or_else(|| Error::DBError("State not found.".to_string()))?
-            .into_beacon_state()
-            .ok_or_else(|| Error::DBError("State invalid.".to_string()))?;
+        debug!("Finding attesatations for new block...");
 
-        debug!("Finding attesatations for block...");
+        let attestations = self
+            .attestation_aggregator
+            .read()
+            .get_attestations_for_state(&state, &self.spec);
 
-        let attestations = {
-            let mut next_state = parent_state.clone();
-            next_state.per_slot_processing(Hash256::zero(), &self.spec)?;
-            self.attestation_aggregator
-                .read()
-                .unwrap()
-                .get_attestations_for_state(&next_state, &self.spec)
-        };
+        debug!(
+            "Inserting {} attestation(s) into new block.",
+            attestations.len()
+        );
 
-        debug!("Found {} attestation(s).", attestations.len());
+        let parent_root = state
+            .get_block_root(state.slot.saturating_sub(1), &self.spec)
+            // TODO: fix unwrap
+            .unwrap()
+            .clone();
 
         let mut block = BeaconBlock {
-            slot: present_slot,
-            parent_root: parent_root.clone(),
+            slot: state.slot,
+            parent_root,
             state_root: Hash256::zero(), // Updated after the state is calculated.
             randao_reveal: randao_reveal,
             eth1_data: Eth1Data {
@@ -86,8 +74,8 @@ where
             },
         };
 
-        let state =
-            self.state_transition_without_verifying_block_signature(parent_state, &block)?;
+        state.per_block_processing_without_verifying_block_signature(&block, &self.spec)?;
+
         let state_root = state.canonical_root();
 
         block.state_root = state_root;
@@ -104,9 +92,15 @@ impl From<DBError> for Error {
     }
 }
 
-impl From<TransitionError> for Error {
-    fn from(e: TransitionError) -> Error {
-        Error::StateTransitionError(e)
+impl From<SlotProcessingError> for Error {
+    fn from(e: SlotProcessingError) -> Error {
+        Error::SlotProcessingError(e)
+    }
+}
+
+impl From<BlockProcessingError> for Error {
+    fn from(e: BlockProcessingError) -> Error {
+        Error::PerBlockProcessingError(e)
     }
 }
 
@@ -116,8 +110,8 @@ impl From<TestingSlotClockError> for Error {
     }
 }
 
-impl From<SlotProcessingError> for Error {
-    fn from(e: SlotProcessingError) -> Error {
-        Error::SlotProcessingError(e)
+impl From<SystemTimeSlotClockError> for Error {
+    fn from(e: SystemTimeSlotClockError) -> Error {
+        Error::SlotClockError(e)
     }
 }
diff --git a/beacon_node/beacon_chain/src/canonical_head.rs b/beacon_node/beacon_chain/src/canonical_head.rs
index a92cff327..14f1adf81 100644
--- a/beacon_node/beacon_chain/src/canonical_head.rs
+++ b/beacon_node/beacon_chain/src/canonical_head.rs
@@ -1,5 +1,5 @@
 use crate::{BeaconChain, CheckPoint, ClientDB, SlotClock};
-use std::sync::RwLockReadGuard;
+use parking_lot::RwLockReadGuard;
 use types::{beacon_state::SlotProcessingError, BeaconBlock, BeaconState, Hash256};
 
 #[derive(Debug, PartialEq)]
@@ -20,10 +20,7 @@ where
         new_beacon_state: BeaconState,
         new_beacon_state_root: Hash256,
     ) {
-        let mut head = self
-            .canonical_head
-            .write()
-            .expect("CRITICAL: CanonicalHead poisioned.");
+        let mut head = self.canonical_head.write();
         head.update(
             new_beacon_block,
             new_beacon_block_root,
@@ -33,34 +30,18 @@ where
     }
 
     pub fn head(&self) -> RwLockReadGuard<CheckPoint> {
-        self.canonical_head
-            .read()
-            .expect("CRITICAL: CanonicalHead poisioned.")
+        self.canonical_head.read()
     }
 
-    pub fn state(&self, slot: u64) -> Result<BeaconState, Error> {
-        let mut state = self
-            .canonical_head
-            .read()
-            .expect("CRITICAL: CanonicalHead poisioned.")
-            .beacon_state
-            .clone();
-        let previous_block_root = self
-            .canonical_head
-            .read()
-            .expect("CRITICAL: CanonicalHead poisioned.")
-            .beacon_block_root
-            .clone();
-
-        match slot.checked_sub(state.slot) {
-            None => Err(Error::PastSlot),
-            Some(distance) => {
-                for _ in 0..distance {
-                    state.per_slot_processing(previous_block_root.clone(), &self.spec)?
-                }
-                Ok(state)
-            }
+    pub fn advance_state(&self, slot: u64) -> Result<(), SlotProcessingError> {
+        let state_slot = self.state.read().slot;
+        let head_block_root = self.head().beacon_block_root;
+        for _ in state_slot..slot {
+            self.state
+                .write()
+                .per_slot_processing(head_block_root.clone(), &self.spec)?;
         }
+        Ok(())
     }
 }
 
diff --git a/beacon_node/beacon_chain/src/finalized_head.rs b/beacon_node/beacon_chain/src/finalized_head.rs
index 28c883b4c..76a20a2c2 100644
--- a/beacon_node/beacon_chain/src/finalized_head.rs
+++ b/beacon_node/beacon_chain/src/finalized_head.rs
@@ -1,5 +1,5 @@
 use crate::{BeaconChain, CheckPoint, ClientDB, SlotClock};
-use std::sync::RwLockReadGuard;
+use parking_lot::RwLockReadGuard;
 use types::{BeaconBlock, BeaconState, Hash256};
 
 impl<T, U> BeaconChain<T, U>
@@ -14,10 +14,7 @@ where
         new_beacon_state: BeaconState,
         new_beacon_state_root: Hash256,
     ) {
-        let mut finalized_head = self
-            .finalized_head
-            .write()
-            .expect("CRITICAL: finalized_head poisioned.");
+        let mut finalized_head = self.finalized_head.write();
         finalized_head.update(
             new_beacon_block,
             new_beacon_block_root,
@@ -27,8 +24,6 @@ where
     }
 
     pub fn finalized_head(&self) -> RwLockReadGuard<CheckPoint> {
-        self.finalized_head
-            .read()
-            .expect("CRITICAL: finalized_head poisioned.")
+        self.finalized_head.read()
     }
 }
diff --git a/beacon_node/beacon_chain/src/info.rs b/beacon_node/beacon_chain/src/info.rs
index e23aaef78..34ac64c8e 100644
--- a/beacon_node/beacon_chain/src/info.rs
+++ b/beacon_node/beacon_chain/src/info.rs
@@ -28,10 +28,7 @@ where
     }
 
     pub fn proposer_slots(&self, validator_index: usize) -> Option<u64> {
-        let slot = self.present_slot()?;
-        let state = self.state(slot).ok()?;
-
-        if let Some(validator) = state.validator_registry.get(validator_index) {
+        if let Some(validator) = self.state.read().validator_registry.get(validator_index) {
             Some(validator.proposer_slots)
         } else {
             None
@@ -46,35 +43,22 @@ where
     }
 
     pub fn block_proposer(&self, slot: u64) -> Result<usize, CommitteesError> {
-        // TODO: fix unwrap
-        let present_slot = self.present_slot().unwrap();
-        // TODO: fix unwrap
-        let state = self.state(present_slot).unwrap();
-        let index = state.get_beacon_proposer_index(slot, &self.spec)?;
+        let index = self
+            .state
+            .read()
+            .get_beacon_proposer_index(slot, &self.spec)?;
 
         Ok(index)
     }
 
     pub fn justified_slot(&self) -> u64 {
-        // TODO: fix unwrap
-        let present_slot = self.present_slot().unwrap();
-        // TODO: fix unwrap
-        let state = self.state(present_slot).unwrap();
-        state.justified_slot
-        /*
-        self.justified_head
-            .read()
-            .expect("Justified head poisoned")
-            .beacon_block
-            .slot
-            */
+        self.state.read().justified_slot
     }
 
     pub fn validator_attestion_slot_and_shard(&self, validator_index: usize) -> Option<(u64, u64)> {
-        let present_slot = self.present_slot()?;
-        let state = self.state(present_slot).ok()?;
-
-        let (slot, shard, _committee) = state
+        let (slot, shard, _committee) = self
+            .state
+            .read()
             .attestation_slot_and_shard_for_validator(validator_index, &self.spec)
             .ok()?;
         Some((slot, shard))
diff --git a/beacon_node/beacon_chain/src/lib.rs b/beacon_node/beacon_chain/src/lib.rs
index 5fdadd34f..5c719dddd 100644
--- a/beacon_node/beacon_chain/src/lib.rs
+++ b/beacon_node/beacon_chain/src/lib.rs
@@ -10,7 +10,7 @@ pub mod dump;
 mod finalized_head;
 mod info;
 mod lmd_ghost;
-mod state_transition;
+// mod state_transition;
 
 use self::attestation_targets::AttestationTargets;
 use self::block_graph::BlockGraph;
@@ -20,9 +20,10 @@ use db::{
     ClientDB, DBError,
 };
 use genesis::{genesis_beacon_block, genesis_beacon_state, GenesisError};
+use parking_lot::RwLock;
 use slot_clock::SlotClock;
 use ssz::ssz_encode;
-use std::sync::{Arc, RwLock};
+use std::sync::Arc;
 use types::{BeaconBlock, BeaconState, ChainSpec, Hash256};
 
 pub use self::block_processing::Outcome as BlockProcessingOutcome;
@@ -79,6 +80,7 @@ pub struct BeaconChain<T: ClientDB + Sized, U: SlotClock> {
     canonical_head: RwLock<CheckPoint>,
     finalized_head: RwLock<CheckPoint>,
     justified_head: RwLock<CheckPoint>,
+    pub state: RwLock<BeaconState>,
     pub latest_attestation_targets: RwLock<AttestationTargets>,
     pub spec: ChainSpec,
 }
@@ -137,6 +139,7 @@ where
             slot_clock,
             block_graph,
             attestation_aggregator,
+            state: RwLock::new(genesis_state.clone()),
             justified_head,
             finalized_head,
             canonical_head,
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 d78a17720..15184e10c 100644
--- a/beacon_node/beacon_chain/test_harness/benches/state_transition.rs
+++ b/beacon_node/beacon_chain/test_harness/benches/state_transition.rs
@@ -1,64 +1,80 @@
 use criterion::Criterion;
-use criterion::{criterion_group, criterion_main};
+use criterion::{black_box, criterion_group, criterion_main, Benchmark};
+use env_logger::{Builder, Env};
 use test_harness::BeaconChainHarness;
-use types::ChainSpec;
+use types::{ChainSpec, Hash256};
 
 fn mid_epoch_state_transition(c: &mut Criterion) {
-    let validator_count = 2;
+    Builder::from_env(Env::default().default_filter_or("debug")).init();
+
+    let validator_count = 1000;
     let mut rig = BeaconChainHarness::new(ChainSpec::foundation(), validator_count);
 
-    let two_and_half_epochs = (rig.spec.epoch_length * 2) + (rig.spec.epoch_length / 2);
+    let epoch_depth = (rig.spec.epoch_length * 2) + (rig.spec.epoch_length / 2);
 
-    for _ in 0..two_and_half_epochs {
+    for _ in 0..epoch_depth {
         rig.advance_chain_with_block();
     }
 
-    let block = rig.advance_chain_without_block();
-    let state = rig.beacon_chain.canonical_head().beacon_state.clone();
+    let state = rig.beacon_chain.state.read().clone();
+
+    assert!((state.slot + 1) % rig.spec.epoch_length != 0);
 
     c.bench_function("mid-epoch state transition 10k validators", move |b| {
-        let block = block.clone();
         let state = state.clone();
         b.iter(|| {
-            rig.beacon_chain
-                .state_transition(state.clone(), &block.clone())
+            let mut state = state.clone();
+            black_box(state.per_slot_processing(Hash256::zero(), &rig.spec))
         })
     });
 }
 
 fn epoch_boundary_state_transition(c: &mut Criterion) {
-    let validator_count = 10_000;
+    // Builder::from_env(Env::default().default_filter_or("debug")).init();
+
+    let validator_count = 10000;
     let mut rig = BeaconChainHarness::new(ChainSpec::foundation(), validator_count);
 
-    let three_epochs = rig.spec.epoch_length * 3;
+    let epoch_depth = rig.spec.epoch_length * 2;
 
-    for _ in 0..(three_epochs - 1) {
+    for _ in 0..(epoch_depth - 1) {
         rig.advance_chain_with_block();
     }
 
-    let state = rig.beacon_chain.canonical_head().beacon_state.clone();
-    assert_eq!(
-        state.slot % rig.spec.epoch_length,
-        rig.spec.epoch_length - 1,
-    );
-    let block = rig.advance_chain_without_block();
+    let state = rig.beacon_chain.state.read().clone();
 
-    c.bench_function("epoch boundary state transition 10k validators", move |b| {
-        let block = block.clone();
+    assert_eq!((state.slot + 1) % rig.spec.epoch_length, 0);
+
+    c.bench(
+        "routines",
+        Benchmark::new("routine_1", move |b| {
+            let state = state.clone();
+            b.iter(|| {
+                let mut state = state.clone();
+                black_box(black_box(
+                    state.per_slot_processing(Hash256::zero(), &rig.spec),
+                ))
+            })
+        })
+        .sample_size(5),
+    );
+
+    /*
+    c.bench_function("mid-epoch state transition 10k validators", move |b| {
         let state = state.clone();
         b.iter(|| {
-            let state = rig
-                .beacon_chain
-                .state_transition(state.clone(), &block.clone())
-                .unwrap();
-            assert_eq!(state.slot % rig.spec.epoch_length, 0);
+            let mut state = state.clone();
+            black_box(black_box(
+                state.per_slot_processing(Hash256::zero(), &rig.spec),
+            ))
         })
     });
+    */
 }
 
 criterion_group!(
     benches,
-    mid_epoch_state_transition,
+    // mid_epoch_state_transition,
     epoch_boundary_state_transition
 );
 criterion_main!(benches);
diff --git a/beacon_node/beacon_chain/test_harness/src/harness.rs b/beacon_node/beacon_chain/test_harness/src/harness.rs
index 3cdfc8024..7d05c7b89 100644
--- a/beacon_node/beacon_chain/test_harness/src/harness.rs
+++ b/beacon_node/beacon_chain/test_harness/src/harness.rs
@@ -113,6 +113,7 @@ impl BeaconChainHarness {
         debug!("Incrementing BeaconChain slot to {}.", slot);
 
         self.beacon_chain.slot_clock.set_slot(slot);
+        self.beacon_chain.advance_state(slot).unwrap();
         slot
     }
 
@@ -125,8 +126,8 @@ impl BeaconChainHarness {
 
         let attesting_validators = self
             .beacon_chain
-            .state(present_slot)
-            .unwrap()
+            .state
+            .read()
             .get_crosslink_committees_at_slot(present_slot, &self.spec)
             .unwrap()
             .iter()
@@ -195,13 +196,27 @@ impl BeaconChainHarness {
         self.beacon_chain.process_block(block).unwrap();
         debug!("...block processed by BeaconChain.");
 
+        debug!("Producing free attestations...");
+
         // Produce new attestations.
         let free_attestations = self.gather_free_attesations();
+
+        debug!("Processing free attestations...");
+
+        free_attestations.par_iter().for_each(|free_attestation| {
+            self.beacon_chain
+                .process_free_attestation(free_attestation.clone())
+                .unwrap();
+        });
+
+        debug!("Free attestations processed.");
+        /*
         for free_attestation in free_attestations {
             self.beacon_chain
                 .process_free_attestation(free_attestation)
                 .unwrap();
         }
+        */
     }
 
     pub fn chain_dump(&self) -> Result<Vec<SlotDump>, DumpError> {
diff --git a/beacon_node/beacon_chain/test_harness/src/validator/beacon_node/attester.rs b/beacon_node/beacon_chain/test_harness/src/validator/beacon_node/attester.rs
index 9c6ce7456..c3e3191f7 100644
--- a/beacon_node/beacon_chain/test_harness/src/validator/beacon_node/attester.rs
+++ b/beacon_node/beacon_chain/test_harness/src/validator/beacon_node/attester.rs
@@ -13,10 +13,10 @@ where
 {
     fn produce_attestation_data(
         &self,
-        slot: u64,
+        _slot: u64,
         shard: u64,
     ) -> Result<Option<AttestationData>, NodeError> {
-        match self.beacon_chain.produce_attestation_data(slot, shard) {
+        match self.beacon_chain.produce_attestation_data(shard) {
             Ok(attestation_data) => Ok(Some(attestation_data)),
             Err(e) => Err(NodeError::RemoteFailure(format!("{:?}", e))),
         }
diff --git a/beacon_node/beacon_chain/test_harness/src/validator/mod.rs b/beacon_node/beacon_chain/test_harness/src/validator/mod.rs
index 018ffbc86..adb2cdc45 100644
--- a/beacon_node/beacon_chain/test_harness/src/validator/mod.rs
+++ b/beacon_node/beacon_chain/test_harness/src/validator/mod.rs
@@ -2,6 +2,7 @@ use attester::{Attester, Error as AttestationPollError};
 use beacon_chain::BeaconChain;
 use block_producer::{BlockProducer, Error as BlockPollError};
 use db::MemoryDB;
+use log::trace;
 use signer::TestSigner;
 use slot_clock::TestingSlotClock;
 use std::sync::Arc;
diff --git a/beacon_node/beacon_chain/test_harness/tests/chain.rs b/beacon_node/beacon_chain/test_harness/tests/chain.rs
index f84157533..ece9bd340 100644
--- a/beacon_node/beacon_chain/test_harness/tests/chain.rs
+++ b/beacon_node/beacon_chain/test_harness/tests/chain.rs
@@ -24,7 +24,7 @@ fn it_can_build_on_genesis_block() {
 fn it_can_produce_past_first_epoch_boundary() {
     Builder::from_env(Env::default().default_filter_or("debug")).init();
 
-    let validator_count = 100;
+    let validator_count = 128 * 1024;
 
     debug!("Starting harness build...");
 
diff --git a/eth2/types/Cargo.toml b/eth2/types/Cargo.toml
index a779c7e7e..77cfb6040 100644
--- a/eth2/types/Cargo.toml
+++ b/eth2/types/Cargo.toml
@@ -12,6 +12,7 @@ hashing = { path = "../utils/hashing" }
 honey-badger-split =  { path = "../utils/honey-badger-split" }
 integer-sqrt = "0.1"
 log = "0.4"
+rayon = "1.0"
 rand = "0.5.5"
 serde = "1.0"
 serde_derive = "1.0"
diff --git a/eth2/types/src/beacon_state/attestation_validation.rs b/eth2/types/src/beacon_state/attestation_validation.rs
index c4a1f0d8a..6e6ade34f 100644
--- a/eth2/types/src/beacon_state/attestation_validation.rs
+++ b/eth2/types/src/beacon_state/attestation_validation.rs
@@ -61,7 +61,7 @@ impl BeaconState {
             attestation.data.slot + spec.epoch_length >= self.slot,
             Error::IncludedTooLate
         );
-        if self.justified_slot >= self.slot - (self.slot % spec.epoch_length) {
+        if attestation.data.slot >= self.current_epoch_start_slot(spec) {
             ensure!(
                 attestation.data.justified_slot == self.justified_slot,
                 Error::WrongJustifiedSlot
diff --git a/beacon_node/beacon_chain/src/state_transition.rs b/eth2/types/src/beacon_state/block_processing.rs
similarity index 66%
rename from beacon_node/beacon_chain/src/state_transition.rs
rename to eth2/types/src/beacon_state/block_processing.rs
index 405da055c..df232d6d1 100644
--- a/beacon_node/beacon_chain/src/state_transition.rs
+++ b/eth2/types/src/beacon_state/block_processing.rs
@@ -1,19 +1,12 @@
-use super::{BeaconChain, ClientDB, DBError, SlotClock};
+use crate::{
+    beacon_state::{AttestationValidationError, CommitteesError, SlotProcessingError},
+    readers::BeaconBlockReader,
+    BeaconBlock, BeaconState, ChainSpec, Exit, Fork, Hash256, PendingAttestation,
+};
 use bls::{PublicKey, Signature};
 use hashing::hash;
 use log::debug;
-use slot_clock::{SystemTimeSlotClockError, TestingSlotClockError};
 use ssz::{ssz_encode, TreeHash};
-use types::{
-    beacon_state::{AttestationValidationError, CommitteesError, SlotProcessingError},
-    readers::BeaconBlockReader,
-    BeaconBlock, BeaconState, Exit, Fork, Hash256, PendingAttestation,
-};
-
-// TODO: define elsehwere.
-const DOMAIN_PROPOSAL: u64 = 2;
-const DOMAIN_EXIT: u64 = 3;
-const DOMAIN_RANDAO: u64 = 4;
 
 macro_rules! ensure {
     ($condition: expr, $result: expr) => {
@@ -23,6 +16,11 @@ macro_rules! ensure {
     };
 }
 
+// TODO: define elsehwere.
+const DOMAIN_PROPOSAL: u64 = 2;
+const DOMAIN_EXIT: u64 = 3;
+const DOMAIN_RANDAO: u64 = 4;
+
 #[derive(Debug, PartialEq)]
 pub enum Error {
     DBError(String),
@@ -50,71 +48,50 @@ pub enum Error {
     BadCustodyReseeds,
     BadCustodyChallenges,
     BadCustodyResponses,
-    SlotClockError(SystemTimeSlotClockError),
     CommitteesError(CommitteesError),
     SlotProcessingError(SlotProcessingError),
 }
 
-impl<T, U> BeaconChain<T, U>
-where
-    T: ClientDB,
-    U: SlotClock,
-{
-    pub fn state_transition(
-        &self,
-        state: BeaconState,
+impl BeaconState {
+    pub fn per_block_processing(
+        &mut self,
         block: &BeaconBlock,
-    ) -> Result<BeaconState, Error> {
-        self.internal_state_transition(state, block, true)
+        spec: &ChainSpec,
+    ) -> Result<(), Error> {
+        self.per_block_processing_signature_optional(block, true, spec)
     }
 
-    pub fn state_transition_without_verifying_block_signature(
-        &self,
-        state: BeaconState,
+    pub fn per_block_processing_without_verifying_block_signature(
+        &mut self,
         block: &BeaconBlock,
-    ) -> Result<BeaconState, Error> {
-        self.internal_state_transition(state, block, false)
+        spec: &ChainSpec,
+    ) -> Result<(), Error> {
+        self.per_block_processing_signature_optional(block, false, spec)
     }
 
-    fn internal_state_transition(
-        &self,
-        mut state: BeaconState,
+    fn per_block_processing_signature_optional(
+        &mut self,
         block: &BeaconBlock,
         verify_block_signature: bool,
-    ) -> Result<BeaconState, Error> {
-        ensure!(state.slot < block.slot, Error::StateAlreadyTransitioned);
-
-        debug!(
-            "Starting state transition from slot {} to {}...",
-            state.slot, block.slot
-        );
-
-        for _ in state.slot..block.slot {
-            state.per_slot_processing(block.parent_root.clone(), &self.spec)?;
-        }
-
-        /*
-         * Slot
-         */
-
-        ensure!(block.slot() == state.slot, Error::StateSlotMismatch);
+        spec: &ChainSpec,
+    ) -> Result<(), Error> {
+        ensure!(block.slot() == self.slot, Error::StateSlotMismatch);
 
         /*
          * Proposer Signature
          */
-
-        let block_proposer_index = state
-            .get_beacon_proposer_index(block.slot, &self.spec)
+        let block_proposer_index = self
+            .get_beacon_proposer_index(block.slot, spec)
             .map_err(|_| Error::NoBlockProducer)?;
-        let block_proposer = &state.validator_registry[block_proposer_index];
+        let block_proposer = &self.validator_registry[block_proposer_index];
 
         if verify_block_signature {
             ensure!(
                 bls_verify(
                     &block_proposer.pubkey,
-                    &block.proposal_root(&self.spec)[..],
+                    &block.proposal_root(spec)[..],
                     &block.signature,
-                    get_domain(&state.fork_data, state.slot, DOMAIN_PROPOSAL)
+                    get_domain(&self.fork_data, self.slot, DOMAIN_PROPOSAL)
                 ),
                 Error::BadBlockSignature
             );
@@ -123,49 +100,42 @@ where
         /*
          * RANDAO
          */
-
         ensure!(
             bls_verify(
                 &block_proposer.pubkey,
                 &ssz_encode(&block_proposer.proposer_slots),
                 &block.randao_reveal,
-                get_domain(&state.fork_data, state.slot, DOMAIN_RANDAO)
+                get_domain(&self.fork_data, self.slot, DOMAIN_RANDAO)
             ),
             Error::BadRandaoSignature
         );
 
         // TODO: check this is correct.
         let new_mix = {
-            let mut mix = state.latest_randao_mixes
-                [(state.slot % self.spec.latest_randao_mixes_length) as usize]
+            let mut mix = self.latest_randao_mixes
+                [(self.slot % spec.latest_randao_mixes_length) as usize]
                 .to_vec();
             mix.append(&mut ssz_encode(&block.randao_reveal));
             Hash256::from(&hash(&mix)[..])
         };
 
-        state.latest_randao_mixes[(state.slot % self.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
          */
 
-        // TODO: Eth1 data stuff.
-
-        /*
-         * OPERATIONS
-         */
+        // TODO: Eth1 data processing.
 
         /*
          * Proposer slashings
          */
-
         ensure!(
-            block.body.proposer_slashings.len() as u64 <= self.spec.max_proposer_slashings,
+            block.body.proposer_slashings.len() as u64 <= spec.max_proposer_slashings,
             Error::MaxProposerSlashingsExceeded
         );
         for proposer_slashing in &block.body.proposer_slashings {
-            let proposer = state
+            let proposer = self
                 .validator_registry
                 .get(proposer_slashing.proposer_index as usize)
                 .ok_or(Error::BadProposerSlashing)?;
@@ -183,7 +153,7 @@ where
                 Error::BadProposerSlashing
             );
             ensure!(
-                proposer.penalized_slot > state.slot,
+                proposer.penalized_slot > self.slot,
                 Error::BadProposerSlashing
             );
             ensure!(
@@ -192,7 +162,7 @@ where
                     &proposer_slashing.proposal_data_1.hash_tree_root(),
                     &proposer_slashing.proposal_signature_1,
                     get_domain(
-                        &state.fork_data,
+                        &self.fork_data,
                         proposer_slashing.proposal_data_1.slot,
                         DOMAIN_PROPOSAL
                     )
@@ -205,34 +175,34 @@ where
                     &proposer_slashing.proposal_data_2.hash_tree_root(),
                     &proposer_slashing.proposal_signature_2,
                     get_domain(
-                        &state.fork_data,
+                        &self.fork_data,
                         proposer_slashing.proposal_data_2.slot,
                         DOMAIN_PROPOSAL
                     )
                 ),
                 Error::BadProposerSlashing
             );
-            penalize_validator(&state, proposer_slashing.proposer_index as usize);
+            penalize_validator(&self, proposer_slashing.proposer_index as usize);
         }
 
         /*
          * Attestations
          */
         ensure!(
-            block.body.attestations.len() as u64 <= self.spec.max_attestations,
+            block.body.attestations.len() as u64 <= spec.max_attestations,
             Error::MaxAttestationsExceeded
         );
 
         for attestation in &block.body.attestations {
-            state.validate_attestation(attestation, &self.spec)?;
+            self.validate_attestation(attestation, spec)?;
 
             let pending_attestation = PendingAttestation {
                 data: attestation.data.clone(),
                 aggregation_bitfield: attestation.aggregation_bitfield.clone(),
                 custody_bitfield: attestation.custody_bitfield.clone(),
-                slot_included: state.slot,
+                slot_included: self.slot,
             };
-            state.latest_attestations.push(pending_attestation);
+            self.latest_attestations.push(pending_attestation);
         }
 
         debug!(
@@ -244,7 +214,7 @@ where
          * Deposits
          */
         ensure!(
-            block.body.deposits.len() as u64 <= self.spec.max_deposits,
+            block.body.deposits.len() as u64 <= spec.max_deposits,
             Error::MaxDepositsExceeded
         );
 
@@ -255,25 +225,25 @@ where
          */
 
         ensure!(
-            block.body.exits.len() as u64 <= self.spec.max_exits,
+            block.body.exits.len() as u64 <= spec.max_exits,
             Error::MaxExitsExceeded
         );
 
         for exit in &block.body.exits {
-            let validator = state
+            let validator = self
                 .validator_registry
                 .get(exit.validator_index as usize)
                 .ok_or(Error::BadExit)?;
             ensure!(
-                validator.exit_slot > state.slot + self.spec.entry_exit_delay,
+                validator.exit_slot > self.slot + spec.entry_exit_delay,
                 Error::BadExit
             );
-            ensure!(state.slot >= exit.slot, Error::BadExit);
+            ensure!(self.slot >= exit.slot, Error::BadExit);
             let exit_message = {
                 let exit_struct = Exit {
                     slot: exit.slot,
                     validator_index: exit.validator_index,
-                    signature: self.spec.empty_signature.clone(),
+                    signature: spec.empty_signature.clone(),
                 };
                 exit_struct.hash_tree_root()
             };
@@ -282,11 +252,11 @@ where
                     &validator.pubkey,
                     &exit_message,
                     &exit.signature,
-                    get_domain(&state.fork_data, exit.slot, DOMAIN_EXIT)
+                    get_domain(&self.fork_data, exit.slot, DOMAIN_EXIT)
                 ),
                 Error::BadProposerSlashing
             );
-            initiate_validator_exit(&state, exit.validator_index);
+            initiate_validator_exit(&self, exit.validator_index);
         }
 
         /*
@@ -307,7 +277,7 @@ where
 
         debug!("State transition complete.");
 
-        Ok(state)
+        Ok(())
     }
 }
 
@@ -329,24 +299,6 @@ fn bls_verify(pubkey: &PublicKey, message: &[u8], signature: &Signature, _domain
     signature.verify(message, pubkey)
 }
 
-impl From<DBError> for Error {
-    fn from(e: DBError) -> Error {
-        Error::DBError(e.message)
-    }
-}
-
-impl From<TestingSlotClockError> for Error {
-    fn from(_: TestingSlotClockError) -> Error {
-        unreachable!(); // Testing clock never throws an error.
-    }
-}
-
-impl From<SystemTimeSlotClockError> for Error {
-    fn from(e: SystemTimeSlotClockError) -> Error {
-        Error::SlotClockError(e)
-    }
-}
-
 impl From<AttestationValidationError> for Error {
     fn from(e: AttestationValidationError) -> Error {
         Error::InvalidAttestation(e)
diff --git a/eth2/types/src/beacon_state/epoch_processing.rs b/eth2/types/src/beacon_state/epoch_processing.rs
index 296c2e848..d93cd4fe5 100644
--- a/eth2/types/src/beacon_state/epoch_processing.rs
+++ b/eth2/types/src/beacon_state/epoch_processing.rs
@@ -7,6 +7,7 @@ use crate::{
 };
 use integer_sqrt::IntegerSquareRoot;
 use log::debug;
+use rayon::prelude::*;
 use std::collections::{HashMap, HashSet};
 use std::iter::FromIterator;
 
@@ -59,7 +60,7 @@ impl BeaconState {
 
         let current_epoch_attestations: Vec<&PendingAttestation> = self
             .latest_attestations
-            .iter()
+            .par_iter()
             .filter(|a| a.data.slot / spec.epoch_length == self.current_epoch(spec))
             .collect();
 
@@ -77,7 +78,7 @@ impl BeaconState {
 
         let current_epoch_boundary_attestations: Vec<&PendingAttestation> =
             current_epoch_attestations
-                .iter()
+                .par_iter()
                 .filter(|a| {
                     match self.get_block_root(self.current_epoch_start_slot(spec), spec) {
                         Some(block_root) => {
@@ -112,7 +113,7 @@ impl BeaconState {
          */
         let previous_epoch_attestations: Vec<&PendingAttestation> = self
             .latest_attestations
-            .iter()
+            .par_iter()
             .filter(|a| {
                 //TODO: ensure these saturating subs are correct.
                 a.data.slot / spec.epoch_length == self.previous_epoch(spec)
diff --git a/eth2/types/src/beacon_state/mod.rs b/eth2/types/src/beacon_state/mod.rs
index 3972eaa82..f0548914e 100644
--- a/eth2/types/src/beacon_state/mod.rs
+++ b/eth2/types/src/beacon_state/mod.rs
@@ -12,6 +12,7 @@ use ssz::{hash, Decodable, DecodeError, Encodable, SszStream, TreeHash};
 
 mod attestation_participants;
 mod attestation_validation;
+mod block_processing;
 mod committees;
 mod epoch_processing;
 mod shuffling;
@@ -20,6 +21,7 @@ mod winning_root;
 
 pub use self::attestation_participants::Error as AttestationParticipantsError;
 pub use self::attestation_validation::Error as AttestationValidationError;
+pub use self::block_processing::Error as BlockProcessingError;
 pub use self::committees::Error as CommitteesError;
 pub use self::epoch_processing::Error as EpochProcessingError;
 pub use self::slot_processing::Error as SlotProcessingError;