diff --git a/beacon_node/beacon_chain/src/beacon_chain.rs b/beacon_node/beacon_chain/src/beacon_chain.rs
index 816a570c0..745ba5155 100644
--- a/beacon_node/beacon_chain/src/beacon_chain.rs
+++ b/beacon_node/beacon_chain/src/beacon_chain.rs
@@ -26,7 +26,10 @@ pub enum ValidBlock {
 #[derive(Debug, PartialEq)]
 pub enum InvalidBlock {
     /// The block slot is greater than the present slot.
-    FutureSlot,
+    FutureSlot {
+        present_slot: Slot,
+        block_slot: Slot,
+    },
     /// The block state_root does not match the generated state.
     StateRootMismatch,
     /// The blocks parent_root is unknown.
@@ -46,6 +49,35 @@ pub enum BlockProcessingOutcome {
     InvalidBlock(InvalidBlock),
 }
 
+impl BlockProcessingOutcome {
+    /// Returns `true` if the block was objectively invalid and we should disregard the peer who
+    /// sent it.
+    pub fn is_invalid(&self) -> bool {
+        match self {
+            BlockProcessingOutcome::ValidBlock(_) => false,
+            BlockProcessingOutcome::InvalidBlock(r) => match r {
+                InvalidBlock::FutureSlot { .. } => true,
+                InvalidBlock::StateRootMismatch => true,
+                InvalidBlock::ParentUnknown => false,
+                InvalidBlock::SlotProcessingError(_) => false,
+                InvalidBlock::PerBlockProcessingError(e) => match e {
+                    BlockProcessingError::Invalid(_) => true,
+                    BlockProcessingError::BeaconStateError(_) => false,
+                },
+            },
+        }
+    }
+
+    /// Returns `true` if the block was successfully processed and can be removed from any import
+    /// queues or temporary storage.
+    pub fn sucessfully_processed(&self) -> bool {
+        match self {
+            BlockProcessingOutcome::ValidBlock(_) => true,
+            _ => false,
+        }
+    }
+}
+
 pub struct BeaconChain<T: ClientDB + Sized, U: SlotClock, F: ForkChoice> {
     pub block_store: Arc<BeaconBlockStore<T>>,
     pub state_store: Arc<BeaconStateStore<T>>,
@@ -122,6 +154,126 @@ where
         })
     }
 
+    /// Returns the beacon block body for each beacon block root in `roots`.
+    ///
+    /// Fails if any root in `roots` does not have a corresponding block.
+    pub fn get_block_bodies(&self, roots: &[Hash256]) -> Result<Vec<BeaconBlockBody>, Error> {
+        let bodies: Result<Vec<BeaconBlockBody>, _> = roots
+            .iter()
+            .map(|root| match self.get_block(root)? {
+                Some(block) => Ok(block.body),
+                None => Err(Error::DBInconsistent("Missing block".into())),
+            })
+            .collect();
+
+        Ok(bodies?)
+    }
+
+    /// Returns the beacon block header for each beacon block root in `roots`.
+    ///
+    /// Fails if any root in `roots` does not have a corresponding block.
+    pub fn get_block_headers(&self, roots: &[Hash256]) -> Result<Vec<BeaconBlockHeader>, Error> {
+        let headers: Result<Vec<BeaconBlockHeader>, _> = roots
+            .iter()
+            .map(|root| match self.get_block(root)? {
+                Some(block) => Ok(block.block_header()),
+                None => Err(Error::DBInconsistent("Missing block".into())),
+            })
+            .collect();
+
+        Ok(headers?)
+    }
+
+    /// Returns `count `beacon block roots, starting from `start_slot` with an
+    /// interval of `skip` slots between each root.
+    ///
+    /// ## Errors:
+    ///
+    /// - `SlotOutOfBounds`: Unable to return the full specified range.
+    /// - `SlotOutOfBounds`: Unable to load a state from the DB.
+    /// - `SlotOutOfBounds`: Start slot is higher than the first slot.
+    /// - Other: BeaconState` is inconsistent.
+    pub fn get_block_roots(
+        &self,
+        earliest_slot: Slot,
+        count: usize,
+        skip: usize,
+    ) -> Result<Vec<Hash256>, Error> {
+        let spec = &self.spec;
+        let step_by = Slot::from(skip + 1);
+
+        let mut roots: Vec<Hash256> = vec![];
+
+        // The state for reading block roots. Will be updated with an older state if slots go too
+        // far back in history.
+        let mut state = self.state.read().clone();
+
+        // The final slot in this series, will be reduced by `skip` each loop iteration.
+        let mut slot = earliest_slot + Slot::from(count * (skip + 1)) - 1;
+
+        // If the highest slot requested is that of the current state insert the root of the
+        // head block, unless the head block's slot is not matching.
+        if slot == state.slot && self.head().beacon_block.slot == slot {
+            roots.push(self.head().beacon_block_root);
+
+            slot -= step_by;
+        } else if slot >= state.slot {
+            return Err(BeaconStateError::SlotOutOfBounds.into());
+        }
+
+        loop {
+            // If the slot is within the range of the current state's block roots, append the root
+            // to the output vec.
+            //
+            // If we get `SlotOutOfBounds` error, load the oldest available historic
+            // state from the DB.
+            match state.get_block_root(slot, spec) {
+                Ok(root) => {
+                    if slot < earliest_slot {
+                        break;
+                    } else {
+                        roots.push(*root);
+                        slot -= step_by;
+                    }
+                }
+                Err(BeaconStateError::SlotOutOfBounds) => {
+                    // Read the earliest historic state in the current slot.
+                    let earliest_historic_slot =
+                        state.slot - Slot::from(spec.slots_per_historical_root);
+                    // Load the earlier state from disk.
+                    let new_state_root = state.get_state_root(earliest_historic_slot, spec)?;
+
+                    // Break if the DB is unable to load the state.
+                    state = match self.state_store.get_deserialized(&new_state_root) {
+                        Ok(Some(state)) => state,
+                        _ => break,
+                    }
+                }
+                Err(e) => return Err(e.into()),
+            };
+        }
+
+        // Return the results if they pass a sanity check.
+        if (slot <= earliest_slot) && (roots.len() == count) {
+            // Reverse the ordering of the roots. We extracted them in reverse order to make it
+            // simpler to lookup historic states.
+            //
+            // This is a potential optimisation target.
+            Ok(roots.iter().rev().cloned().collect())
+        } else {
+            Err(BeaconStateError::SlotOutOfBounds.into())
+        }
+    }
+
+    /// Returns the block at the given root, if any.
+    ///
+    /// ## Errors
+    ///
+    /// May return a database error.
+    pub fn get_block(&self, block_root: &Hash256) -> Result<Option<BeaconBlock>, Error> {
+        Ok(self.block_store.get_deserialized(block_root)?)
+    }
+
     /// Update the canonical head to some new values.
     pub fn update_canonical_head(
         &self,
@@ -153,6 +305,49 @@ where
         self.canonical_head.read()
     }
 
+    /// Updates the canonical `BeaconState` with the supplied state.
+    ///
+    /// Advances the chain forward to the present slot. This method is better than just setting
+    /// state and calling `catchup_state` as it will not result in an old state being installed and
+    /// then having it iteratively updated -- in such a case it's possible for another thread to
+    /// find the state at an old slot.
+    pub fn update_state(&self, mut state: BeaconState) -> Result<(), Error> {
+        let latest_block_header = self.head().beacon_block.block_header();
+
+        let present_slot = match self.slot_clock.present_slot() {
+            Ok(Some(slot)) => slot,
+            _ => return Err(Error::UnableToReadSlot),
+        };
+
+        // If required, transition the new state to the present slot.
+        for _ in state.slot.as_u64()..present_slot.as_u64() {
+            per_slot_processing(&mut state, &latest_block_header, &self.spec)?;
+        }
+
+        *self.state.write() = state;
+
+        Ok(())
+    }
+
+    /// Ensures the current canonical `BeaconState` has been transitioned to match the `slot_clock`.
+    pub fn catchup_state(&self) -> Result<(), Error> {
+        let latest_block_header = self.head().beacon_block.block_header();
+
+        let present_slot = match self.slot_clock.present_slot() {
+            Ok(Some(slot)) => slot,
+            _ => return Err(Error::UnableToReadSlot),
+        };
+
+        let mut state = self.state.write();
+
+        // If required, transition the new state to the present slot.
+        for _ in state.slot.as_u64()..present_slot.as_u64() {
+            per_slot_processing(&mut *state, &latest_block_header, &self.spec)?;
+        }
+
+        Ok(())
+    }
+
     /// Update the justified head to some new values.
     pub fn update_finalized_head(
         &self,
@@ -176,28 +371,6 @@ where
         self.finalized_head.read()
     }
 
-    /// Advance the `self.state` `BeaconState` to the supplied slot.
-    ///
-    /// This will perform per_slot and per_epoch processing as required.
-    ///
-    /// The `previous_block_root` will be set to the root of the current head block (as determined
-    /// by the fork-choice rule).
-    ///
-    /// It is important to note that this is _not_ the state corresponding to the canonical head
-    /// block, instead it is that state which may or may not have had additional per slot/epoch
-    /// processing applied to it.
-    pub fn advance_state(&self, slot: Slot) -> Result<(), SlotProcessingError> {
-        let state_slot = self.state.read().slot;
-
-        let latest_block_header = self.head().beacon_block.block_header();
-
-        for _ in state_slot.as_u64()..slot.as_u64() {
-            per_slot_processing(&mut *self.state.write(), &latest_block_header, &self.spec)?;
-        }
-
-        Ok(())
-    }
-
     /// Returns the validator index (if any) for the given public key.
     ///
     /// Information is retrieved from the present `beacon_state.validator_registry`.
@@ -246,7 +419,10 @@ where
     /// Information is read from the present `beacon_state` shuffling, so only information from the
     /// present and prior epoch is available.
     pub fn block_proposer(&self, slot: Slot) -> Result<usize, BeaconStateError> {
-        trace!("BeaconChain::block_proposer: slot: {}", slot);
+        self.state
+            .write()
+            .build_epoch_cache(RelativeEpoch::Current, &self.spec)?;
+
         let index = self.state.read().get_beacon_proposer_index(
             slot,
             RelativeEpoch::Current,
@@ -555,6 +731,11 @@ where
         }
     }
 
+    /// Returns `true` if the given block root has not been processed.
+    pub fn is_new_block_root(&self, beacon_block_root: &Hash256) -> Result<bool, Error> {
+        Ok(!self.block_store.exists(beacon_block_root)?)
+    }
+
     /// Accept some block and attempt to add it to block DAG.
     ///
     /// Will accept blocks from prior slots, however it will reject any block from a future slot.
@@ -567,7 +748,10 @@ where
 
         if block.slot > present_slot {
             return Ok(BlockProcessingOutcome::InvalidBlock(
-                InvalidBlock::FutureSlot,
+                InvalidBlock::FutureSlot {
+                    present_slot,
+                    block_slot: block.slot,
+                },
             ));
         }
 
@@ -594,10 +778,10 @@ where
         // TODO: check the block proposer signature BEFORE doing a state transition. This will
         // significantly lower exposure surface to DoS attacks.
 
-        // Transition the parent state to the present slot.
+        // Transition the parent state to the block slot.
         let mut state = parent_state;
         let previous_block_header = parent_block.block_header();
-        for _ in state.slot.as_u64()..present_slot.as_u64() {
+        for _ in state.slot.as_u64()..block.slot.as_u64() {
             if let Err(e) = per_slot_processing(&mut state, &previous_block_header, &self.spec) {
                 return Ok(BlockProcessingOutcome::InvalidBlock(
                     InvalidBlock::SlotProcessingError(e),
@@ -643,8 +827,9 @@ where
         // run instead.
         if self.head().beacon_block_root == parent_block_root {
             self.update_canonical_head(block.clone(), block_root, state.clone(), state_root);
-            // Update the local state variable.
-            *self.state.write() = state;
+
+            // Update the canonical `BeaconState`.
+            self.update_state(state)?;
         }
 
         Ok(BlockProcessingOutcome::ValidBlock(ValidBlock::Processed))
@@ -662,6 +847,8 @@ where
 
         let mut state = self.state.read().clone();
 
+        state.build_epoch_cache(RelativeEpoch::Current, &self.spec)?;
+
         trace!("Finding attestations for new block...");
 
         let attestations = self
@@ -732,7 +919,10 @@ where
                 .ok_or_else(|| Error::MissingBeaconState(block.state_root))?;
             let state_root = state.canonical_root();
 
-            self.update_canonical_head(block, block_root, state, state_root);
+            self.update_canonical_head(block, block_root, state.clone(), state_root);
+
+            // Update the canonical `BeaconState`.
+            self.update_state(state)?;
         }
 
         Ok(())
diff --git a/beacon_node/beacon_chain/src/checkpoint.rs b/beacon_node/beacon_chain/src/checkpoint.rs
index 828e462de..78227e5c8 100644
--- a/beacon_node/beacon_chain/src/checkpoint.rs
+++ b/beacon_node/beacon_chain/src/checkpoint.rs
@@ -3,7 +3,7 @@ use types::{BeaconBlock, BeaconState, Hash256};
 
 /// Represents some block and it's associated state. Generally, this will be used for tracking the
 /// head, justified head and finalized head.
-#[derive(Clone, Serialize)]
+#[derive(Clone, Serialize, PartialEq, Debug)]
 pub struct CheckPoint {
     pub beacon_block: BeaconBlock,
     pub beacon_block_root: Hash256,
diff --git a/beacon_node/beacon_chain/src/errors.rs b/beacon_node/beacon_chain/src/errors.rs
index 58c3f87ae..a84e4b10e 100644
--- a/beacon_node/beacon_chain/src/errors.rs
+++ b/beacon_node/beacon_chain/src/errors.rs
@@ -1,5 +1,6 @@
 use fork_choice::ForkChoiceError;
 use state_processing::BlockProcessingError;
+use state_processing::SlotProcessingError;
 use types::*;
 
 macro_rules! easy_from_to {
@@ -16,18 +17,24 @@ macro_rules! easy_from_to {
 pub enum BeaconChainError {
     InsufficientValidators,
     BadRecentBlockRoots,
+    UnableToReadSlot,
     BeaconStateError(BeaconStateError),
     DBInconsistent(String),
     DBError(String),
     ForkChoiceError(ForkChoiceError),
     MissingBeaconBlock(Hash256),
     MissingBeaconState(Hash256),
+    SlotProcessingError(SlotProcessingError),
 }
 
+easy_from_to!(SlotProcessingError, BeaconChainError);
+
 #[derive(Debug, PartialEq)]
 pub enum BlockProductionError {
     UnableToGetBlockRootFromState,
     BlockProcessingError(BlockProcessingError),
+    BeaconStateError(BeaconStateError),
 }
 
 easy_from_to!(BlockProcessingError, BlockProductionError);
+easy_from_to!(BeaconStateError, BlockProductionError);
diff --git a/beacon_node/beacon_chain/src/lib.rs b/beacon_node/beacon_chain/src/lib.rs
index 2137c0edf..48a42b941 100644
--- a/beacon_node/beacon_chain/src/lib.rs
+++ b/beacon_node/beacon_chain/src/lib.rs
@@ -3,10 +3,12 @@ mod beacon_chain;
 mod checkpoint;
 mod errors;
 pub mod initialise;
+pub mod test_utils;
 
 pub use self::beacon_chain::{BeaconChain, BlockProcessingOutcome, InvalidBlock, ValidBlock};
 pub use self::checkpoint::CheckPoint;
 pub use self::errors::BeaconChainError;
+pub use attestation_aggregator::Outcome as AggregationOutcome;
 pub use db;
 pub use fork_choice;
 pub use parking_lot;
diff --git a/beacon_node/beacon_chain/src/test_utils/mod.rs b/beacon_node/beacon_chain/src/test_utils/mod.rs
new file mode 100644
index 000000000..ad251a3c9
--- /dev/null
+++ b/beacon_node/beacon_chain/src/test_utils/mod.rs
@@ -0,0 +1,3 @@
+mod testing_beacon_chain_builder;
+
+pub use testing_beacon_chain_builder::TestingBeaconChainBuilder;
diff --git a/beacon_node/beacon_chain/src/test_utils/testing_beacon_chain_builder.rs b/beacon_node/beacon_chain/src/test_utils/testing_beacon_chain_builder.rs
new file mode 100644
index 000000000..5c5477e55
--- /dev/null
+++ b/beacon_node/beacon_chain/src/test_utils/testing_beacon_chain_builder.rs
@@ -0,0 +1,50 @@
+pub use crate::{BeaconChain, BeaconChainError, CheckPoint};
+use db::{
+    stores::{BeaconBlockStore, BeaconStateStore},
+    MemoryDB,
+};
+use fork_choice::BitwiseLMDGhost;
+use slot_clock::TestingSlotClock;
+use ssz::TreeHash;
+use std::sync::Arc;
+use types::test_utils::TestingBeaconStateBuilder;
+use types::*;
+
+type TestingBeaconChain = BeaconChain<MemoryDB, TestingSlotClock, BitwiseLMDGhost<MemoryDB>>;
+
+pub struct TestingBeaconChainBuilder {
+    state_builder: TestingBeaconStateBuilder,
+}
+
+impl TestingBeaconChainBuilder {
+    pub fn build(self, spec: &ChainSpec) -> TestingBeaconChain {
+        let db = Arc::new(MemoryDB::open());
+        let block_store = Arc::new(BeaconBlockStore::new(db.clone()));
+        let state_store = Arc::new(BeaconStateStore::new(db.clone()));
+        let slot_clock = TestingSlotClock::new(spec.genesis_slot.as_u64());
+        let fork_choice = BitwiseLMDGhost::new(block_store.clone(), state_store.clone());
+
+        let (genesis_state, _keypairs) = self.state_builder.build();
+
+        let mut genesis_block = BeaconBlock::empty(&spec);
+        genesis_block.state_root = Hash256::from_slice(&genesis_state.hash_tree_root());
+
+        // Create the Beacon Chain
+        BeaconChain::from_genesis(
+            state_store.clone(),
+            block_store.clone(),
+            slot_clock,
+            genesis_state,
+            genesis_block,
+            spec.clone(),
+            fork_choice,
+        )
+        .unwrap()
+    }
+}
+
+impl From<TestingBeaconStateBuilder> for TestingBeaconChainBuilder {
+    fn from(state_builder: TestingBeaconStateBuilder) -> TestingBeaconChainBuilder {
+        TestingBeaconChainBuilder { state_builder }
+    }
+}
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 bc5c93b94..1498796b1 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
@@ -15,6 +15,8 @@ use std::iter::FromIterator;
 use std::sync::Arc;
 use types::{test_utils::TestingBeaconStateBuilder, *};
 
+type TestingBeaconChain = BeaconChain<MemoryDB, TestingSlotClock, BitwiseLMDGhost<MemoryDB>>;
+
 /// The beacon chain harness simulates a single beacon node with `validator_count` validators connected
 /// to it. Each validator is provided a borrow to the beacon chain, where it may read
 /// information and submit blocks/attestations for processing.
@@ -23,7 +25,7 @@ use types::{test_utils::TestingBeaconStateBuilder, *};
 /// is not useful for testing that multiple beacon nodes can reach consensus.
 pub struct BeaconChainHarness {
     pub db: Arc<MemoryDB>,
-    pub beacon_chain: Arc<BeaconChain<MemoryDB, TestingSlotClock, BitwiseLMDGhost<MemoryDB>>>,
+    pub beacon_chain: Arc<TestingBeaconChain>,
     pub block_store: Arc<BeaconBlockStore<MemoryDB>>,
     pub state_store: Arc<BeaconStateStore<MemoryDB>>,
     pub validators: Vec<ValidatorHarness>,
@@ -36,19 +38,39 @@ impl BeaconChainHarness {
     /// - A keypair, `BlockProducer` and `Attester` for each validator.
     /// - A new BeaconChain struct where the given validators are in the genesis.
     pub fn new(spec: ChainSpec, validator_count: usize) -> Self {
+        let state_builder =
+            TestingBeaconStateBuilder::from_default_keypairs_file_if_exists(validator_count, &spec);
+        Self::from_beacon_state_builder(state_builder, spec)
+    }
+
+    pub fn from_beacon_state_builder(
+        state_builder: TestingBeaconStateBuilder,
+        spec: ChainSpec,
+    ) -> Self {
         let db = Arc::new(MemoryDB::open());
         let block_store = Arc::new(BeaconBlockStore::new(db.clone()));
         let state_store = Arc::new(BeaconStateStore::new(db.clone()));
         let slot_clock = TestingSlotClock::new(spec.genesis_slot.as_u64());
         let fork_choice = BitwiseLMDGhost::new(block_store.clone(), state_store.clone());
 
-        let state_builder =
-            TestingBeaconStateBuilder::from_default_keypairs_file_if_exists(validator_count, &spec);
-        let (genesis_state, keypairs) = state_builder.build();
+        let (mut genesis_state, keypairs) = state_builder.build();
 
         let mut genesis_block = BeaconBlock::empty(&spec);
         genesis_block.state_root = Hash256::from_slice(&genesis_state.hash_tree_root());
 
+        genesis_state
+            .build_epoch_cache(RelativeEpoch::Previous, &spec)
+            .unwrap();
+        genesis_state
+            .build_epoch_cache(RelativeEpoch::Current, &spec)
+            .unwrap();
+        genesis_state
+            .build_epoch_cache(RelativeEpoch::NextWithoutRegistryChange, &spec)
+            .unwrap();
+        genesis_state
+            .build_epoch_cache(RelativeEpoch::NextWithRegistryChange, &spec)
+            .unwrap();
+
         // Create the Beacon Chain
         let beacon_chain = Arc::new(
             BeaconChain::from_genesis(
@@ -109,7 +131,9 @@ impl BeaconChainHarness {
         );
 
         self.beacon_chain.slot_clock.set_slot(slot.as_u64());
-        self.beacon_chain.advance_state(slot).unwrap();
+        self.beacon_chain
+            .catchup_state()
+            .expect("Failed to catch state");
         slot
     }
 
@@ -187,7 +211,6 @@ impl BeaconChainHarness {
         self.increment_beacon_chain_slot();
 
         // Produce a new block.
-        debug!("Producing block...");
         let block = self.produce_block();
         debug!("Submitting block for processing...");
         match self.beacon_chain.process_block(block) {
diff --git a/beacon_node/eth2-libp2p/Cargo.toml b/beacon_node/eth2-libp2p/Cargo.toml
index f3914b04e..d9c43b23c 100644
--- a/beacon_node/eth2-libp2p/Cargo.toml
+++ b/beacon_node/eth2-libp2p/Cargo.toml
@@ -5,6 +5,7 @@ authors = ["Age Manning <Age@AgeManning.com>"]
 edition = "2018"
 
 [dependencies]
+beacon_chain =  { path = "../beacon_chain" }
 # SigP repository until PR is merged
 libp2p =  { git = "https://github.com/SigP/rust-libp2p", rev = "b3c32d9a821ae6cc89079499cc6e8a6bab0bffc3" }
 types = { path =  "../../eth2/types" }
diff --git a/beacon_node/eth2-libp2p/src/behaviour.rs b/beacon_node/eth2-libp2p/src/behaviour.rs
index 458b32cf9..3d5b94353 100644
--- a/beacon_node/eth2-libp2p/src/behaviour.rs
+++ b/beacon_node/eth2-libp2p/src/behaviour.rs
@@ -1,3 +1,4 @@
+use crate::rpc::methods::BlockRootSlot;
 use crate::rpc::{RPCEvent, RPCMessage, Rpc};
 use crate::NetworkConfig;
 use futures::prelude::*;
@@ -13,6 +14,8 @@ use libp2p::{
     NetworkBehaviour, PeerId,
 };
 use slog::{debug, o};
+use ssz_derive::{Decode, Encode};
+use types::Attestation;
 use types::Topic;
 
 /// Builds the network behaviour for the libp2p Swarm.
@@ -154,3 +157,21 @@ pub enum BehaviourEvent {
     // TODO: This is a stub at the moment
     Message(String),
 }
+
+#[derive(Debug, Clone)]
+pub enum IncomingGossip {
+    Block(BlockGossip),
+    Attestation(AttestationGossip),
+}
+
+/// Gossipsub message providing notification of a new block.
+#[derive(Encode, Decode, Clone, Debug, PartialEq)]
+pub struct BlockGossip {
+    pub root: BlockRootSlot,
+}
+
+/// Gossipsub message providing notification of a new attestation.
+#[derive(Encode, Decode, Clone, Debug, PartialEq)]
+pub struct AttestationGossip {
+    pub attestation: Attestation,
+}
diff --git a/beacon_node/eth2-libp2p/src/rpc/methods.rs b/beacon_node/eth2-libp2p/src/rpc/methods.rs
index 3014afd0f..ad3233be7 100644
--- a/beacon_node/eth2-libp2p/src/rpc/methods.rs
+++ b/beacon_node/eth2-libp2p/src/rpc/methods.rs
@@ -1,6 +1,7 @@
+use ssz::{Decodable, DecodeError, Encodable, SszStream};
 /// Available RPC methods types and ids.
 use ssz_derive::{Decode, Encode};
-use types::{BeaconBlockBody, BeaconBlockHeader, Epoch, Hash256, Slot};
+use types::{Attestation, BeaconBlockBody, BeaconBlockHeader, Epoch, Hash256, Slot};
 
 #[derive(Debug)]
 /// Available Serenity Libp2p RPC methods
@@ -53,13 +54,27 @@ impl Into<u16> for RPCMethod {
 #[derive(Debug, Clone)]
 pub enum RPCRequest {
     Hello(HelloMessage),
-    Goodbye(u64),
+    Goodbye(GoodbyeReason),
     BeaconBlockRoots(BeaconBlockRootsRequest),
     BeaconBlockHeaders(BeaconBlockHeadersRequest),
     BeaconBlockBodies(BeaconBlockBodiesRequest),
     BeaconChainState(BeaconChainStateRequest),
 }
 
+impl RPCRequest {
+    pub fn method_id(&self) -> u16 {
+        let method = match self {
+            RPCRequest::Hello(_) => RPCMethod::Hello,
+            RPCRequest::Goodbye(_) => RPCMethod::Goodbye,
+            RPCRequest::BeaconBlockRoots(_) => RPCMethod::BeaconBlockRoots,
+            RPCRequest::BeaconBlockHeaders(_) => RPCMethod::BeaconBlockHeaders,
+            RPCRequest::BeaconBlockBodies(_) => RPCMethod::BeaconBlockBodies,
+            RPCRequest::BeaconChainState(_) => RPCMethod::BeaconChainState,
+        };
+        method.into()
+    }
+}
+
 #[derive(Debug, Clone)]
 pub enum RPCResponse {
     Hello(HelloMessage),
@@ -69,6 +84,19 @@ pub enum RPCResponse {
     BeaconChainState(BeaconChainStateResponse),
 }
 
+impl RPCResponse {
+    pub fn method_id(&self) -> u16 {
+        let method = match self {
+            RPCResponse::Hello(_) => RPCMethod::Hello,
+            RPCResponse::BeaconBlockRoots(_) => RPCMethod::BeaconBlockRoots,
+            RPCResponse::BeaconBlockHeaders(_) => RPCMethod::BeaconBlockHeaders,
+            RPCResponse::BeaconBlockBodies(_) => RPCMethod::BeaconBlockBodies,
+            RPCResponse::BeaconChainState(_) => RPCMethod::BeaconChainState,
+        };
+        method.into()
+    }
+}
+
 /* Request/Response data structures for RPC methods */
 
 /// The HELLO request/response handshake message.
@@ -86,76 +114,125 @@ pub struct HelloMessage {
     pub best_slot: Slot,
 }
 
+/// The reason given for a `Goodbye` message.
+///
+/// Note: any unknown `u64::into(n)` will resolve to `GoodbyeReason::Unknown` for any unknown `n`,
+/// however `GoodbyeReason::Unknown.into()` will go into `0_u64`. Therefore de-serializing then
+/// re-serializing may not return the same bytes.
+#[derive(Debug, Clone)]
+pub enum GoodbyeReason {
+    ClientShutdown,
+    IrreleventNetwork,
+    Fault,
+    Unknown,
+}
+
+impl From<u64> for GoodbyeReason {
+    fn from(id: u64) -> GoodbyeReason {
+        match id {
+            1 => GoodbyeReason::ClientShutdown,
+            2 => GoodbyeReason::IrreleventNetwork,
+            3 => GoodbyeReason::Fault,
+            _ => GoodbyeReason::Unknown,
+        }
+    }
+}
+
+impl Into<u64> for GoodbyeReason {
+    fn into(self) -> u64 {
+        match self {
+            GoodbyeReason::Unknown => 0,
+            GoodbyeReason::ClientShutdown => 1,
+            GoodbyeReason::IrreleventNetwork => 2,
+            GoodbyeReason::Fault => 3,
+        }
+    }
+}
+
+impl Encodable for GoodbyeReason {
+    fn ssz_append(&self, s: &mut SszStream) {
+        let id: u64 = (*self).clone().into();
+        id.ssz_append(s);
+    }
+}
+
+impl Decodable for GoodbyeReason {
+    fn ssz_decode(bytes: &[u8], index: usize) -> Result<(Self, usize), DecodeError> {
+        let (id, index) = u64::ssz_decode(bytes, index)?;
+        Ok((Self::from(id), index))
+    }
+}
+
 /// Request a number of beacon block roots from a peer.
-#[derive(Encode, Decode, Clone, Debug)]
+#[derive(Encode, Decode, Clone, Debug, PartialEq)]
 pub struct BeaconBlockRootsRequest {
     /// The starting slot of the requested blocks.
-    start_slot: Slot,
+    pub start_slot: Slot,
     /// The number of blocks from the start slot.
-    count: u64, // this must be less than 32768. //TODO: Enforce this in the lower layers
+    pub count: u64, // this must be less than 32768. //TODO: Enforce this in the lower layers
 }
 
 /// Response containing a number of beacon block roots from a peer.
-#[derive(Encode, Decode, Clone, Debug)]
+#[derive(Encode, Decode, Clone, Debug, PartialEq)]
 pub struct BeaconBlockRootsResponse {
     /// List of requested blocks and associated slots.
-    roots: Vec<BlockRootSlot>,
+    pub roots: Vec<BlockRootSlot>,
 }
 
 /// Contains a block root and associated slot.
-#[derive(Encode, Decode, Clone, Debug)]
+#[derive(Encode, Decode, Clone, Debug, PartialEq)]
 pub struct BlockRootSlot {
     /// The block root.
-    block_root: Hash256,
+    pub block_root: Hash256,
     /// The block slot.
-    slot: Slot,
+    pub slot: Slot,
 }
 
 /// Request a number of beacon block headers from a peer.
-#[derive(Encode, Decode, Clone, Debug)]
+#[derive(Encode, Decode, Clone, Debug, PartialEq)]
 pub struct BeaconBlockHeadersRequest {
     /// The starting header hash of the requested headers.
-    start_root: Hash256,
+    pub start_root: Hash256,
     /// The starting slot of the requested headers.
-    start_slot: Slot,
+    pub start_slot: Slot,
     /// The maximum number of headers than can be returned.
-    max_headers: u64,
+    pub max_headers: u64,
     /// The maximum number of slots to skip between blocks.
-    skip_slots: u64,
+    pub skip_slots: u64,
 }
 
 /// Response containing requested block headers.
-#[derive(Encode, Decode, Clone, Debug)]
+#[derive(Encode, Decode, Clone, Debug, PartialEq)]
 pub struct BeaconBlockHeadersResponse {
     /// The list of requested beacon block headers.
-    headers: Vec<BeaconBlockHeader>,
+    pub headers: Vec<BeaconBlockHeader>,
 }
 
 /// Request a number of beacon block bodies from a peer.
-#[derive(Encode, Decode, Clone, Debug)]
+#[derive(Encode, Decode, Clone, Debug, PartialEq)]
 pub struct BeaconBlockBodiesRequest {
     /// The list of beacon block bodies being requested.
-    block_roots: Hash256,
+    pub block_roots: Vec<Hash256>,
 }
 
 /// Response containing the list of requested beacon block bodies.
-#[derive(Encode, Decode, Clone, Debug)]
+#[derive(Encode, Decode, Clone, Debug, PartialEq)]
 pub struct BeaconBlockBodiesResponse {
     /// The list of beacon block bodies being requested.
-    block_bodies: Vec<BeaconBlockBody>,
+    pub block_bodies: Vec<BeaconBlockBody>,
 }
 
 /// Request values for tree hashes which yield a blocks `state_root`.
-#[derive(Encode, Decode, Clone, Debug)]
+#[derive(Encode, Decode, Clone, Debug, PartialEq)]
 pub struct BeaconChainStateRequest {
     /// The tree hashes that a value is requested for.
-    hashes: Vec<Hash256>,
+    pub hashes: Vec<Hash256>,
 }
 
 /// Request values for tree hashes which yield a blocks `state_root`.
 // Note: TBD
-#[derive(Encode, Decode, Clone, Debug)]
+#[derive(Encode, Decode, Clone, Debug, PartialEq)]
 pub struct BeaconChainStateResponse {
     /// The values corresponding the to the requested tree hashes.
-    values: bool, //TBD - stubbed with encodeable bool
+    pub values: bool, //TBD - stubbed with encodeable bool
 }
diff --git a/beacon_node/eth2-libp2p/src/rpc/mod.rs b/beacon_node/eth2-libp2p/src/rpc/mod.rs
index a1cfadafe..08573aa52 100644
--- a/beacon_node/eth2-libp2p/src/rpc/mod.rs
+++ b/beacon_node/eth2-libp2p/src/rpc/mod.rs
@@ -2,7 +2,7 @@
 ///
 /// This is purpose built for Ethereum 2.0 serenity and the protocol listens on
 /// `/eth/serenity/rpc/1.0.0`
-mod methods;
+pub mod methods;
 mod protocol;
 
 use futures::prelude::*;
@@ -12,7 +12,7 @@ use libp2p::core::swarm::{
 };
 use libp2p::{Multiaddr, PeerId};
 pub use methods::{HelloMessage, RPCMethod, RPCRequest, RPCResponse};
-pub use protocol::{RPCEvent, RPCProtocol};
+pub use protocol::{RPCEvent, RPCProtocol, RequestId};
 use slog::o;
 use std::marker::PhantomData;
 use tokio::io::{AsyncRead, AsyncWrite};
diff --git a/beacon_node/eth2-libp2p/src/rpc/protocol.rs b/beacon_node/eth2-libp2p/src/rpc/protocol.rs
index dc3f70a9b..314be1037 100644
--- a/beacon_node/eth2-libp2p/src/rpc/protocol.rs
+++ b/beacon_node/eth2-libp2p/src/rpc/protocol.rs
@@ -1,6 +1,7 @@
 use super::methods::*;
 use libp2p::core::{upgrade, InboundUpgrade, OutboundUpgrade, UpgradeInfo};
-use ssz::{ssz_encode, Decodable, Encodable, SszStream};
+use ssz::{ssz_encode, Decodable, DecodeError as SSZDecodeError, Encodable, SszStream};
+use std::hash::{Hash, Hasher};
 use std::io;
 use std::iter;
 use tokio::io::{AsyncRead, AsyncWrite};
@@ -29,16 +30,65 @@ impl Default for RPCProtocol {
     }
 }
 
+/// A monotonic counter for ordering `RPCRequest`s.
+#[derive(Debug, Clone, PartialEq, Default)]
+pub struct RequestId(u64);
+
+impl RequestId {
+    /// Increment the request id.
+    pub fn increment(&mut self) {
+        self.0 += 1
+    }
+
+    /// Return the previous id.
+    pub fn previous(&self) -> Self {
+        Self(self.0 - 1)
+    }
+}
+
+impl Eq for RequestId {}
+
+impl Hash for RequestId {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.0.hash(state);
+    }
+}
+
+impl From<u64> for RequestId {
+    fn from(x: u64) -> RequestId {
+        RequestId(x)
+    }
+}
+
+impl Into<u64> for RequestId {
+    fn into(self) -> u64 {
+        self.0
+    }
+}
+
+impl Encodable for RequestId {
+    fn ssz_append(&self, s: &mut SszStream) {
+        self.0.ssz_append(s);
+    }
+}
+
+impl Decodable for RequestId {
+    fn ssz_decode(bytes: &[u8], index: usize) -> Result<(Self, usize), SSZDecodeError> {
+        let (id, index) = u64::ssz_decode(bytes, index)?;
+        Ok((Self::from(id), index))
+    }
+}
+
 /// The RPC types which are sent/received in this protocol.
 #[derive(Debug, Clone)]
 pub enum RPCEvent {
     Request {
-        id: u64,
+        id: RequestId,
         method_id: u16,
         body: RPCRequest,
     },
     Response {
-        id: u64,
+        id: RequestId,
         method_id: u16, //TODO: Remove and process decoding upstream
         result: RPCResponse,
     },
@@ -75,7 +125,7 @@ fn decode(packet: Vec<u8>) -> Result<RPCEvent, DecodeError> {
     // decode the header of the rpc
     // request/response
     let (request, index) = bool::ssz_decode(&packet, 0)?;
-    let (id, index) = u64::ssz_decode(&packet, index)?;
+    let (id, index) = RequestId::ssz_decode(&packet, index)?;
     let (method_id, index) = u16::ssz_decode(&packet, index)?;
 
     if request {
@@ -85,8 +135,8 @@ fn decode(packet: Vec<u8>) -> Result<RPCEvent, DecodeError> {
                 RPCRequest::Hello(hello_body)
             }
             RPCMethod::Goodbye => {
-                let (goodbye_code, _index) = u64::ssz_decode(&packet, index)?;
-                RPCRequest::Goodbye(goodbye_code)
+                let (goodbye_reason, _index) = GoodbyeReason::ssz_decode(&packet, index)?;
+                RPCRequest::Goodbye(goodbye_reason)
             }
             RPCMethod::BeaconBlockRoots => {
                 let (block_roots_request, _index) =
diff --git a/beacon_node/network/Cargo.toml b/beacon_node/network/Cargo.toml
index 5275ed82f..c6411a020 100644
--- a/beacon_node/network/Cargo.toml
+++ b/beacon_node/network/Cargo.toml
@@ -4,12 +4,17 @@ version = "0.1.0"
 authors = ["Age Manning <Age@AgeManning.com>"]
 edition = "2018"
 
+[dev-dependencies]
+test_harness = { path = "../beacon_chain/test_harness" }
+sloggers = "0.3.2"
+
 [dependencies]
 beacon_chain =  { path = "../beacon_chain" }
 eth2-libp2p =  { path = "../eth2-libp2p" }
 version = { path = "../version" }
 types = { path = "../../eth2/types" }
 slog = "2.4.1"
+ssz = { path = "../../eth2/utils/ssz" }
 futures = "0.1.25"
 error-chain = "0.12.0"
 crossbeam-channel = "0.3.8"
diff --git a/beacon_node/network/src/beacon_chain.rs b/beacon_node/network/src/beacon_chain.rs
index 91628cc7e..26cea0065 100644
--- a/beacon_node/network/src/beacon_chain.rs
+++ b/beacon_node/network/src/beacon_chain.rs
@@ -5,8 +5,12 @@ use beacon_chain::{
     parking_lot::RwLockReadGuard,
     slot_clock::SlotClock,
     types::{BeaconState, ChainSpec},
-    CheckPoint,
+    AggregationOutcome, CheckPoint,
 };
+use eth2_libp2p::HelloMessage;
+use types::{Attestation, BeaconBlock, BeaconBlockBody, BeaconBlockHeader, Epoch, Hash256, Slot};
+
+pub use beacon_chain::{BeaconChainError, BlockProcessingOutcome};
 
 /// The network's API to the beacon chain.
 pub trait BeaconChain: Send + Sync {
@@ -14,9 +18,48 @@ pub trait BeaconChain: Send + Sync {
 
     fn get_state(&self) -> RwLockReadGuard<BeaconState>;
 
+    fn slot(&self) -> Slot;
+
     fn head(&self) -> RwLockReadGuard<CheckPoint>;
 
+    fn get_block(&self, block_root: &Hash256) -> Result<Option<BeaconBlock>, BeaconChainError>;
+
+    fn best_slot(&self) -> Slot;
+
+    fn best_block_root(&self) -> Hash256;
+
     fn finalized_head(&self) -> RwLockReadGuard<CheckPoint>;
+
+    fn finalized_epoch(&self) -> Epoch;
+
+    fn hello_message(&self) -> HelloMessage;
+
+    fn process_block(&self, block: BeaconBlock)
+        -> Result<BlockProcessingOutcome, BeaconChainError>;
+
+    fn process_attestation(
+        &self,
+        attestation: Attestation,
+    ) -> Result<AggregationOutcome, BeaconChainError>;
+
+    fn get_block_roots(
+        &self,
+        start_slot: Slot,
+        count: usize,
+        skip: usize,
+    ) -> Result<Vec<Hash256>, BeaconChainError>;
+
+    fn get_block_headers(
+        &self,
+        start_slot: Slot,
+        count: usize,
+        skip: usize,
+    ) -> Result<Vec<BeaconBlockHeader>, BeaconChainError>;
+
+    fn get_block_bodies(&self, roots: &[Hash256])
+        -> Result<Vec<BeaconBlockBody>, BeaconChainError>;
+
+    fn is_new_block_root(&self, beacon_block_root: &Hash256) -> Result<bool, BeaconChainError>;
 }
 
 impl<T, U, F> BeaconChain for RawBeaconChain<T, U, F>
@@ -33,11 +76,93 @@ where
         self.state.read()
     }
 
+    fn slot(&self) -> Slot {
+        self.get_state().slot
+    }
+
     fn head(&self) -> RwLockReadGuard<CheckPoint> {
         self.head()
     }
 
+    fn get_block(&self, block_root: &Hash256) -> Result<Option<BeaconBlock>, BeaconChainError> {
+        self.get_block(block_root)
+    }
+
+    fn finalized_epoch(&self) -> Epoch {
+        self.get_state().finalized_epoch
+    }
+
     fn finalized_head(&self) -> RwLockReadGuard<CheckPoint> {
         self.finalized_head()
     }
+
+    fn best_slot(&self) -> Slot {
+        self.head().beacon_block.slot
+    }
+
+    fn best_block_root(&self) -> Hash256 {
+        self.head().beacon_block_root
+    }
+
+    fn hello_message(&self) -> HelloMessage {
+        let spec = self.get_spec();
+        let state = self.get_state();
+
+        HelloMessage {
+            network_id: spec.network_id,
+            latest_finalized_root: state.finalized_root,
+            latest_finalized_epoch: state.finalized_epoch,
+            best_root: self.best_block_root(),
+            best_slot: self.best_slot(),
+        }
+    }
+
+    fn process_block(
+        &self,
+        block: BeaconBlock,
+    ) -> Result<BlockProcessingOutcome, BeaconChainError> {
+        self.process_block(block)
+    }
+
+    fn process_attestation(
+        &self,
+        _attestation: Attestation,
+    ) -> Result<AggregationOutcome, BeaconChainError> {
+        // Awaiting a proper operations pool before we can import attestations.
+        //
+        // Returning a useless error for now.
+        //
+        // https://github.com/sigp/lighthouse/issues/281
+        return Err(BeaconChainError::DBInconsistent("CANNOT PROCESS".into()));
+    }
+
+    fn get_block_roots(
+        &self,
+        start_slot: Slot,
+        count: usize,
+        skip: usize,
+    ) -> Result<Vec<Hash256>, BeaconChainError> {
+        self.get_block_roots(start_slot, count, skip)
+    }
+
+    fn get_block_headers(
+        &self,
+        start_slot: Slot,
+        count: usize,
+        skip: usize,
+    ) -> Result<Vec<BeaconBlockHeader>, BeaconChainError> {
+        let roots = self.get_block_roots(start_slot, count, skip)?;
+        self.get_block_headers(&roots)
+    }
+
+    fn get_block_bodies(
+        &self,
+        roots: &[Hash256],
+    ) -> Result<Vec<BeaconBlockBody>, BeaconChainError> {
+        self.get_block_bodies(roots)
+    }
+
+    fn is_new_block_root(&self, beacon_block_root: &Hash256) -> Result<bool, BeaconChainError> {
+        self.is_new_block_root(beacon_block_root)
+    }
 }
diff --git a/beacon_node/network/src/lib.rs b/beacon_node/network/src/lib.rs
index 61a29ed35..87f8368a5 100644
--- a/beacon_node/network/src/lib.rs
+++ b/beacon_node/network/src/lib.rs
@@ -1,8 +1,8 @@
 /// This crate provides the network server for Lighthouse.
 pub mod beacon_chain;
 pub mod error;
-mod message_handler;
-mod service;
+pub mod message_handler;
+pub mod service;
 pub mod sync;
 
 pub use eth2_libp2p::NetworkConfig;
diff --git a/beacon_node/network/src/message_handler.rs b/beacon_node/network/src/message_handler.rs
index dbf8c7d9d..dcfee96a0 100644
--- a/beacon_node/network/src/message_handler.rs
+++ b/beacon_node/network/src/message_handler.rs
@@ -4,33 +4,29 @@ use crate::service::{NetworkMessage, OutgoingMessage};
 use crate::sync::SimpleSync;
 use crossbeam_channel::{unbounded as channel, Sender};
 use eth2_libp2p::{
-    rpc::{RPCMethod, RPCRequest, RPCResponse},
-    HelloMessage, PeerId, RPCEvent,
+    behaviour::IncomingGossip,
+    rpc::{methods::GoodbyeReason, RPCRequest, RPCResponse, RequestId},
+    PeerId, RPCEvent,
 };
 use futures::future;
-use slog::warn;
-use slog::{debug, trace};
+use slog::{debug, warn};
 use std::collections::HashMap;
 use std::sync::Arc;
-use std::time::{Duration, Instant};
+use std::time::Instant;
 
 /// Timeout for RPC requests.
-const REQUEST_TIMEOUT: Duration = Duration::from_secs(30);
+// const REQUEST_TIMEOUT: Duration = Duration::from_secs(30);
 /// Timeout before banning a peer for non-identification.
-const HELLO_TIMEOUT: Duration = Duration::from_secs(30);
+// const HELLO_TIMEOUT: Duration = Duration::from_secs(30);
 
 /// Handles messages received from the network and client and organises syncing.
 pub struct MessageHandler {
     /// Currently loaded and initialised beacon chain.
-    chain: Arc<BeaconChain>,
+    _chain: Arc<BeaconChain>,
     /// The syncing framework.
     sync: SimpleSync,
-    /// The network channel to relay messages to the Network service.
-    network_send: crossbeam_channel::Sender<NetworkMessage>,
-    /// A mapping of peers and the RPC id we have sent an RPC request to.
-    requests: HashMap<(PeerId, u64), Instant>,
-    /// A counter of request id for each peer.
-    request_ids: HashMap<PeerId, u64>,
+    /// The context required to send messages to, and process messages from peers.
+    network_context: NetworkContext,
     /// The `MessageHandler` logger.
     log: slog::Logger,
 }
@@ -44,8 +40,8 @@ pub enum HandlerMessage {
     PeerDisconnected(PeerId),
     /// An RPC response/request has been received.
     RPC(PeerId, RPCEvent),
-    /// A block has been imported.
-    BlockImported(), //TODO: This comes from pub-sub - decide its contents
+    /// A gossip message has been received.
+    IncomingGossip(PeerId, IncomingGossip),
 }
 
 impl MessageHandler {
@@ -65,13 +61,9 @@ impl MessageHandler {
         let sync = SimpleSync::new(beacon_chain.clone(), &log);
 
         let mut handler = MessageHandler {
-            // TODO: The handler may not need a chain, perhaps only sync?
-            chain: beacon_chain.clone(),
+            _chain: beacon_chain.clone(),
             sync,
-            network_send,
-            requests: HashMap::new(),
-            request_ids: HashMap::new(),
-
+            network_context: NetworkContext::new(network_send, log.clone()),
             log: log.clone(),
         };
 
@@ -93,13 +85,16 @@ impl MessageHandler {
         match message {
             // we have initiated a connection to a peer
             HandlerMessage::PeerDialed(peer_id) => {
-                let id = self.generate_request_id(&peer_id);
-                self.send_hello(peer_id, id, true);
+                self.sync.on_connect(peer_id, &mut self.network_context);
             }
             // we have received an RPC message request/response
             HandlerMessage::RPC(peer_id, rpc_event) => {
                 self.handle_rpc_message(peer_id, rpc_event);
             }
+            // we have received an RPC message request/response
+            HandlerMessage::IncomingGossip(peer_id, gossip) => {
+                self.handle_gossip(peer_id, gossip);
+            }
             //TODO: Handle all messages
             _ => {}
         }
@@ -117,109 +112,195 @@ impl MessageHandler {
     }
 
     /// A new RPC request has been received from the network.
-    fn handle_rpc_request(&mut self, peer_id: PeerId, id: u64, request: RPCRequest) {
+    fn handle_rpc_request(&mut self, peer_id: PeerId, request_id: RequestId, request: RPCRequest) {
+        // TODO: process the `id`.
         match request {
-            RPCRequest::Hello(hello_message) => {
-                self.handle_hello_request(peer_id, id, hello_message)
+            RPCRequest::Hello(hello_message) => self.sync.on_hello_request(
+                peer_id,
+                request_id,
+                hello_message,
+                &mut self.network_context,
+            ),
+            RPCRequest::Goodbye(goodbye_reason) => self.sync.on_goodbye(peer_id, goodbye_reason),
+            RPCRequest::BeaconBlockRoots(request) => self.sync.on_beacon_block_roots_request(
+                peer_id,
+                request_id,
+                request,
+                &mut self.network_context,
+            ),
+            RPCRequest::BeaconBlockHeaders(request) => self.sync.on_beacon_block_headers_request(
+                peer_id,
+                request_id,
+                request,
+                &mut self.network_context,
+            ),
+            RPCRequest::BeaconBlockBodies(request) => self.sync.on_beacon_block_bodies_request(
+                peer_id,
+                request_id,
+                request,
+                &mut self.network_context,
+            ),
+            RPCRequest::BeaconChainState(_) => {
+                // We do not implement this endpoint, it is not required and will only likely be
+                // useful for light-client support in later phases.
+                warn!(self.log, "BeaconChainState RPC call is not supported.");
             }
-            // TODO: Handle all requests
-            _ => {}
         }
     }
 
     /// An RPC response has been received from the network.
     // we match on id and ignore responses past the timeout.
-    fn handle_rpc_response(&mut self, peer_id: PeerId, id: u64, response: RPCResponse) {
-        // if response id is related to a request, ignore (likely RPC timeout)
-        if self.requests.remove(&(peer_id.clone(), id)).is_none() {
-            debug!(self.log, "Unrecognized response from peer: {:?}", peer_id);
+    fn handle_rpc_response(&mut self, peer_id: PeerId, id: RequestId, response: RPCResponse) {
+        // if response id is not related to a request, ignore (likely RPC timeout)
+        if self
+            .network_context
+            .outstanding_outgoing_request_ids
+            .remove(&(peer_id.clone(), id.clone()))
+            .is_none()
+        {
+            warn!(
+                self.log,
+                "Unknown ResponseId for incoming RPCRequest";
+                "peer" => format!("{:?}", peer_id),
+                "request_id" => format!("{:?}", id)
+            );
             return;
         }
+
         match response {
             RPCResponse::Hello(hello_message) => {
-                debug!(self.log, "Hello response received from peer: {:?}", peer_id);
-                self.validate_hello(peer_id, hello_message);
+                self.sync
+                    .on_hello_response(peer_id, hello_message, &mut self.network_context);
+            }
+            RPCResponse::BeaconBlockRoots(response) => {
+                self.sync.on_beacon_block_roots_response(
+                    peer_id,
+                    response,
+                    &mut self.network_context,
+                );
+            }
+            RPCResponse::BeaconBlockHeaders(response) => {
+                self.sync.on_beacon_block_headers_response(
+                    peer_id,
+                    response,
+                    &mut self.network_context,
+                );
+            }
+            RPCResponse::BeaconBlockBodies(response) => {
+                self.sync.on_beacon_block_bodies_response(
+                    peer_id,
+                    response,
+                    &mut self.network_context,
+                );
+            }
+            RPCResponse::BeaconChainState(_) => {
+                // We do not implement this endpoint, it is not required and will only likely be
+                // useful for light-client support in later phases.
+                //
+                // Theoretically, we shouldn't reach this code because we should never send a
+                // beacon state RPC request.
+                warn!(self.log, "BeaconChainState RPC call is not supported.");
             }
-            // TODO: Handle all responses
-            _ => {}
-        }
-    }
-
-    /// Handle a HELLO RPC request message.
-    fn handle_hello_request(&mut self, peer_id: PeerId, id: u64, hello_message: HelloMessage) {
-        // send back a HELLO message
-        self.send_hello(peer_id.clone(), id, false);
-        // validate the peer
-        self.validate_hello(peer_id, hello_message);
-    }
-
-    /// Validate a HELLO RPC message.
-    fn validate_hello(&mut self, peer_id: PeerId, message: HelloMessage) {
-        // validate the peer
-        if !self.sync.validate_peer(peer_id.clone(), message) {
-            debug!(
-                self.log,
-                "Peer dropped due to mismatching HELLO messages: {:?}", peer_id
-            );
-            //TODO: block/ban the peer
-        }
-    }
-
-    /* General RPC helper functions */
-
-    /// Generates a new request id for a peer.
-    fn generate_request_id(&mut self, peer_id: &PeerId) -> u64 {
-        // generate a unique id for the peer
-        let id = {
-            let borrowed_id = self.request_ids.entry(peer_id.clone()).or_insert_with(|| 0);
-            let id = borrowed_id.clone();
-            //increment the counter
-            *borrowed_id += 1;
-            id
         };
-        // register RPC request
-        self.requests.insert((peer_id.clone(), id), Instant::now());
-        debug!(
-            self.log,
-            "Hello request registered with peer: {:?}", peer_id
-        );
-        id
     }
 
-    /// Sends a HELLO RPC request or response to a newly connected peer.
-    //TODO: The boolean determines if sending request/respond, will be cleaner in the RPC re-write
-    fn send_hello(&mut self, peer_id: PeerId, id: u64, is_request: bool) {
-        let rpc_event = if is_request {
+    /// Handle RPC messages
+    fn handle_gossip(&mut self, peer_id: PeerId, gossip_message: IncomingGossip) {
+        match gossip_message {
+            IncomingGossip::Block(message) => {
+                self.sync
+                    .on_block_gossip(peer_id, message, &mut self.network_context)
+            }
+            IncomingGossip::Attestation(message) => {
+                self.sync
+                    .on_attestation_gossip(peer_id, message, &mut self.network_context)
+            }
+        }
+    }
+}
+
+pub struct NetworkContext {
+    /// The network channel to relay messages to the Network service.
+    network_send: crossbeam_channel::Sender<NetworkMessage>,
+    /// A mapping of peers and the RPC id we have sent an RPC request to.
+    outstanding_outgoing_request_ids: HashMap<(PeerId, RequestId), Instant>,
+    /// Stores the next `RequestId` we should include on an outgoing `RPCRequest` to a `PeerId`.
+    outgoing_request_ids: HashMap<PeerId, RequestId>,
+    /// The `MessageHandler` logger.
+    log: slog::Logger,
+}
+
+impl NetworkContext {
+    pub fn new(network_send: crossbeam_channel::Sender<NetworkMessage>, log: slog::Logger) -> Self {
+        Self {
+            network_send,
+            outstanding_outgoing_request_ids: HashMap::new(),
+            outgoing_request_ids: HashMap::new(),
+            log,
+        }
+    }
+
+    pub fn disconnect(&mut self, peer_id: PeerId, reason: GoodbyeReason) {
+        self.send_rpc_request(peer_id, RPCRequest::Goodbye(reason))
+        // TODO: disconnect peers.
+    }
+
+    pub fn send_rpc_request(&mut self, peer_id: PeerId, rpc_request: RPCRequest) {
+        let id = self.generate_request_id(&peer_id);
+
+        self.outstanding_outgoing_request_ids
+            .insert((peer_id.clone(), id.clone()), Instant::now());
+
+        self.send_rpc_event(
+            peer_id,
             RPCEvent::Request {
                 id,
-                method_id: RPCMethod::Hello.into(),
-                body: RPCRequest::Hello(self.sync.generate_hello()),
-            }
-        } else {
-            RPCEvent::Response {
-                id,
-                method_id: RPCMethod::Hello.into(),
-                result: RPCResponse::Hello(self.sync.generate_hello()),
-            }
-        };
-
-        // send the hello request to the network
-        trace!(self.log, "Sending HELLO message to peer {:?}", peer_id);
-        self.send_rpc(peer_id, rpc_event);
+                method_id: rpc_request.method_id(),
+                body: rpc_request,
+            },
+        );
     }
 
-    /// Sends an RPC request/response to the network server.
-    fn send_rpc(&self, peer_id: PeerId, rpc_event: RPCEvent) {
+    pub fn send_rpc_response(
+        &mut self,
+        peer_id: PeerId,
+        request_id: RequestId,
+        rpc_response: RPCResponse,
+    ) {
+        self.send_rpc_event(
+            peer_id,
+            RPCEvent::Response {
+                id: request_id,
+                method_id: rpc_response.method_id(),
+                result: rpc_response,
+            },
+        );
+    }
+
+    fn send_rpc_event(&self, peer_id: PeerId, rpc_event: RPCEvent) {
+        self.send(peer_id, OutgoingMessage::RPC(rpc_event))
+    }
+
+    fn send(&self, peer_id: PeerId, outgoing_message: OutgoingMessage) {
         self.network_send
-            .send(NetworkMessage::Send(
-                peer_id,
-                OutgoingMessage::RPC(rpc_event),
-            ))
+            .send(NetworkMessage::Send(peer_id, outgoing_message))
             .unwrap_or_else(|_| {
                 warn!(
                     self.log,
                     "Could not send RPC message to the network service"
                 )
             });
+        //
+    }
+
+    /// Returns the next `RequestId` for sending an `RPCRequest` to the `peer_id`.
+    fn generate_request_id(&mut self, peer_id: &PeerId) -> RequestId {
+        let next_id = self
+            .outgoing_request_ids
+            .entry(peer_id.clone())
+            .and_modify(|id| id.increment())
+            .or_insert_with(|| RequestId::from(1));
+
+        next_id.previous()
     }
 }
diff --git a/beacon_node/network/src/sync/import_queue.rs b/beacon_node/network/src/sync/import_queue.rs
new file mode 100644
index 000000000..17cbd2f12
--- /dev/null
+++ b/beacon_node/network/src/sync/import_queue.rs
@@ -0,0 +1,232 @@
+use crate::beacon_chain::BeaconChain;
+use eth2_libp2p::rpc::methods::*;
+use eth2_libp2p::PeerId;
+use slog::{debug, error};
+use ssz::TreeHash;
+use std::sync::Arc;
+use std::time::{Duration, Instant};
+use types::{BeaconBlock, BeaconBlockBody, BeaconBlockHeader, Hash256};
+
+/// Provides a queue for fully and partially built `BeaconBlock`s.
+///
+/// The queue is fundamentally a `Vec<PartialBeaconBlock>` where no two items have the same
+/// `item.block_root`. This struct it backed by a `Vec` not a `HashMap` for the following two
+/// reasons:
+///
+/// - When we receive a `BeaconBlockBody`, the only way we can find it's matching
+/// `BeaconBlockHeader` is to find a header such that `header.beacon_block_body ==
+/// hash_tree_root(body)`. Therefore, if we used a `HashMap` we would need to use the root of
+/// `BeaconBlockBody` as the key.
+/// - It is possible for multiple distinct blocks to have identical `BeaconBlockBodies`. Therefore
+/// we cannot use a `HashMap` keyed by the root of `BeaconBlockBody`.
+pub struct ImportQueue {
+    pub chain: Arc<BeaconChain>,
+    /// Partially imported blocks, keyed by the root of `BeaconBlockBody`.
+    pub partials: Vec<PartialBeaconBlock>,
+    /// Time before a queue entry is considered state.
+    pub stale_time: Duration,
+    /// Logging
+    log: slog::Logger,
+}
+
+impl ImportQueue {
+    /// Return a new, empty queue.
+    pub fn new(chain: Arc<BeaconChain>, stale_time: Duration, log: slog::Logger) -> Self {
+        Self {
+            chain,
+            partials: vec![],
+            stale_time,
+            log,
+        }
+    }
+
+    /// Completes all possible partials into `BeaconBlock` and returns them, sorted by increasing
+    /// slot number.  Does not delete the partials from the queue, this must be done manually.
+    ///
+    /// Returns `(queue_index, block, sender)`:
+    ///
+    /// - `block_root`: may be used to remove the entry if it is successfully processed.
+    /// - `block`: the completed block.
+    /// - `sender`: the `PeerId` the provided the `BeaconBlockBody` which completed the partial.
+    pub fn complete_blocks(&self) -> Vec<(Hash256, BeaconBlock, PeerId)> {
+        let mut complete: Vec<(Hash256, BeaconBlock, PeerId)> = self
+            .partials
+            .iter()
+            .filter_map(|partial| partial.clone().complete())
+            .collect();
+
+        // Sort the completable partials to be in ascending slot order.
+        complete.sort_unstable_by(|a, b| a.1.slot.partial_cmp(&b.1.slot).unwrap());
+
+        complete
+    }
+
+    /// Removes the first `PartialBeaconBlock` with a matching `block_root`, returning the partial
+    /// if it exists.
+    pub fn remove(&mut self, block_root: Hash256) -> Option<PartialBeaconBlock> {
+        let position = self
+            .partials
+            .iter()
+            .position(|p| p.block_root == block_root)?;
+        Some(self.partials.remove(position))
+    }
+
+    /// Flushes all stale entries from the queue.
+    ///
+    /// An entry is stale if it has as a `inserted` time that is more than `self.stale_time` in the
+    /// past.
+    pub fn remove_stale(&mut self) {
+        let stale_indices: Vec<usize> = self
+            .partials
+            .iter()
+            .enumerate()
+            .filter_map(|(i, partial)| {
+                if partial.inserted + self.stale_time <= Instant::now() {
+                    Some(i)
+                } else {
+                    None
+                }
+            })
+            .collect();
+
+        if !stale_indices.is_empty() {
+            debug!(
+                self.log,
+                "ImportQueue removing stale entries";
+                "stale_items" => stale_indices.len(),
+                "stale_time_seconds" => self.stale_time.as_secs()
+            );
+        }
+
+        stale_indices.iter().for_each(|&i| {
+            self.partials.remove(i);
+        });
+    }
+
+    /// Returns `true` if `self.chain` has not yet processed this block.
+    pub fn is_new_block(&self, block_root: &Hash256) -> bool {
+        self.chain
+            .is_new_block_root(&block_root)
+            .unwrap_or_else(|_| {
+                error!(self.log, "Unable to determine if block is new.");
+                true
+            })
+    }
+
+    /// Returns the index of the first new root in the list of block roots.
+    pub fn first_new_root(&mut self, roots: &[BlockRootSlot]) -> Option<usize> {
+        roots
+            .iter()
+            .position(|brs| self.is_new_block(&brs.block_root))
+    }
+
+    /// Adds the `headers` to the `partials` queue. Returns a list of `Hash256` block roots for
+    /// which we should use to request `BeaconBlockBodies`.
+    ///
+    /// If a `header` is not in the queue and has not been processed by the chain it is added to
+    /// the queue and it's block root is included in the output.
+    ///
+    /// If a `header` is already in the queue, but not yet processed by the chain the block root is
+    /// included in the output and the `inserted` time for the partial record is set to
+    /// `Instant::now()`. Updating the `inserted` time stops the partial from becoming stale.
+    ///
+    /// Presently the queue enforces that a `BeaconBlockHeader` _must_ be received before its
+    /// `BeaconBlockBody`. This is not a natural requirement and we could enhance the queue to lift
+    /// this restraint.
+    pub fn enqueue_headers(
+        &mut self,
+        headers: Vec<BeaconBlockHeader>,
+        sender: PeerId,
+    ) -> Vec<Hash256> {
+        let mut required_bodies: Vec<Hash256> = vec![];
+
+        for header in headers {
+            let block_root = Hash256::from_slice(&header.hash_tree_root()[..]);
+
+            if self.is_new_block(&block_root) {
+                self.insert_header(block_root, header, sender.clone());
+                required_bodies.push(block_root)
+            }
+        }
+
+        required_bodies
+    }
+
+    /// If there is a matching `header` for this `body`, adds it to the queue.
+    ///
+    /// If there is no `header` for the `body`, the body is simply discarded.
+    pub fn enqueue_bodies(&mut self, bodies: Vec<BeaconBlockBody>, sender: PeerId) {
+        for body in bodies {
+            self.insert_body(body, sender.clone());
+        }
+    }
+
+    /// Inserts a header to the queue.
+    ///
+    /// If the header already exists, the `inserted` time is set to `now` and not other
+    /// modifications are made.
+    fn insert_header(&mut self, block_root: Hash256, header: BeaconBlockHeader, sender: PeerId) {
+        if let Some(i) = self
+            .partials
+            .iter()
+            .position(|p| p.block_root == block_root)
+        {
+            self.partials[i].inserted = Instant::now();
+        } else {
+            self.partials.push(PartialBeaconBlock {
+                block_root,
+                header,
+                body: None,
+                inserted: Instant::now(),
+                sender,
+            })
+        }
+    }
+
+    /// Updates an existing partial with the `body`.
+    ///
+    /// If there is no header for the `body`, the body is simply discarded.
+    ///
+    /// If the body already existed, the `inserted` time is set to `now`.
+    fn insert_body(&mut self, body: BeaconBlockBody, sender: PeerId) {
+        let body_root = Hash256::from_slice(&body.hash_tree_root()[..]);
+
+        self.partials.iter_mut().for_each(|mut p| {
+            if body_root == p.header.block_body_root {
+                p.inserted = Instant::now();
+
+                if p.body.is_none() {
+                    p.body = Some(body.clone());
+                    p.sender = sender.clone();
+                }
+            }
+        });
+    }
+}
+
+/// Individual components of a `BeaconBlock`, potentially all that are required to form a full
+/// `BeaconBlock`.
+#[derive(Clone, Debug)]
+pub struct PartialBeaconBlock {
+    /// `BeaconBlock` root.
+    pub block_root: Hash256,
+    pub header: BeaconBlockHeader,
+    pub body: Option<BeaconBlockBody>,
+    /// The instant at which this record was created or last meaningfully modified. Used to
+    /// determine if an entry is stale and should be removed.
+    pub inserted: Instant,
+    /// The `PeerId` that last meaningfully contributed to this item.
+    pub sender: PeerId,
+}
+
+impl PartialBeaconBlock {
+    /// Consumes `self` and returns a full built `BeaconBlock`, it's root and the `sender`
+    /// `PeerId`, if enough information exists to complete the block. Otherwise, returns `None`.
+    pub fn complete(self) -> Option<(Hash256, BeaconBlock, PeerId)> {
+        Some((
+            self.block_root,
+            self.header.into_block(self.body?),
+            self.sender,
+        ))
+    }
+}
diff --git a/beacon_node/network/src/sync/mod.rs b/beacon_node/network/src/sync/mod.rs
index 8f5216b85..fac1b46eb 100644
--- a/beacon_node/network/src/sync/mod.rs
+++ b/beacon_node/network/src/sync/mod.rs
@@ -1,3 +1,4 @@
+mod import_queue;
 /// Syncing for lighthouse.
 ///
 /// Stores the various syncing methods for the beacon chain.
diff --git a/beacon_node/network/src/sync/simple_sync.rs b/beacon_node/network/src/sync/simple_sync.rs
index 0f7de6ab9..4c08a6871 100644
--- a/beacon_node/network/src/sync/simple_sync.rs
+++ b/beacon_node/network/src/sync/simple_sync.rs
@@ -1,112 +1,686 @@
+use super::import_queue::ImportQueue;
 use crate::beacon_chain::BeaconChain;
-use eth2_libp2p::rpc::HelloMessage;
+use crate::message_handler::NetworkContext;
+use eth2_libp2p::behaviour::{AttestationGossip, BlockGossip};
+use eth2_libp2p::rpc::methods::*;
+use eth2_libp2p::rpc::{RPCRequest, RPCResponse, RequestId};
 use eth2_libp2p::PeerId;
-use slog::{debug, o};
+use slog::{debug, error, info, o, warn};
 use std::collections::HashMap;
 use std::sync::Arc;
+use std::time::Duration;
 use types::{Epoch, Hash256, Slot};
 
 /// The number of slots that we can import blocks ahead of us, before going into full Sync mode.
 const SLOT_IMPORT_TOLERANCE: u64 = 100;
 
+/// The amount of seconds a block (or partial block) may exist in the import queue.
+const QUEUE_STALE_SECS: u64 = 60;
+
 /// Keeps track of syncing information for known connected peers.
+#[derive(Clone, Copy, Debug)]
 pub struct PeerSyncInfo {
+    network_id: u8,
     latest_finalized_root: Hash256,
     latest_finalized_epoch: Epoch,
     best_root: Hash256,
     best_slot: Slot,
 }
 
+impl PeerSyncInfo {
+    /// Returns `true` if the has a different network ID to `other`.
+    fn has_different_network_id_to(&self, other: Self) -> bool {
+        self.network_id != other.network_id
+    }
+
+    /// Returns `true` if the peer has a higher finalized epoch than `other`.
+    fn has_higher_finalized_epoch_than(&self, other: Self) -> bool {
+        self.latest_finalized_epoch > other.latest_finalized_epoch
+    }
+
+    /// Returns `true` if the peer has a higher best slot than `other`.
+    fn has_higher_best_slot_than(&self, other: Self) -> bool {
+        self.best_slot > other.best_slot
+    }
+}
+
+/// The status of a peers view on the chain, relative to some other view of the chain (presumably
+/// our view).
+#[derive(PartialEq, Clone, Copy, Debug)]
+pub enum PeerStatus {
+    /// The peer is on a completely different chain.
+    DifferentNetworkId,
+    /// The peer lists a finalized epoch for which we have a different root.
+    FinalizedEpochNotInChain,
+    /// The peer has a higher finalized epoch.
+    HigherFinalizedEpoch,
+    /// The peer has a higher best slot.
+    HigherBestSlot,
+    /// The peer has the same or lesser view of the chain. We have nothing to request of them.
+    NotInteresting,
+}
+
+impl PeerStatus {
+    pub fn should_handshake(&self) -> bool {
+        match self {
+            PeerStatus::DifferentNetworkId => false,
+            PeerStatus::FinalizedEpochNotInChain => false,
+            PeerStatus::HigherFinalizedEpoch => true,
+            PeerStatus::HigherBestSlot => true,
+            PeerStatus::NotInteresting => true,
+        }
+    }
+}
+
+impl From<HelloMessage> for PeerSyncInfo {
+    fn from(hello: HelloMessage) -> PeerSyncInfo {
+        PeerSyncInfo {
+            network_id: hello.network_id,
+            latest_finalized_root: hello.latest_finalized_root,
+            latest_finalized_epoch: hello.latest_finalized_epoch,
+            best_root: hello.best_root,
+            best_slot: hello.best_slot,
+        }
+    }
+}
+
+impl From<&Arc<BeaconChain>> for PeerSyncInfo {
+    fn from(chain: &Arc<BeaconChain>) -> PeerSyncInfo {
+        Self::from(chain.hello_message())
+    }
+}
+
 /// The current syncing state.
 #[derive(PartialEq)]
 pub enum SyncState {
     Idle,
     Downloading,
-    Stopped,
+    _Stopped,
 }
 
 /// Simple Syncing protocol.
-//TODO: Decide for HELLO messages whether its better to keep current in RAM or build on the fly
-//when asked.
 pub struct SimpleSync {
     /// A reference to the underlying beacon chain.
     chain: Arc<BeaconChain>,
     /// A mapping of Peers to their respective PeerSyncInfo.
     known_peers: HashMap<PeerId, PeerSyncInfo>,
+    /// A queue to allow importing of blocks
+    import_queue: ImportQueue,
     /// The current state of the syncing protocol.
     state: SyncState,
-    /// The network id, for quick HELLO RPC message lookup.
-    network_id: u8,
-    /// The latest epoch of the syncing chain.
-    latest_finalized_epoch: Epoch,
-    /// The latest block of the syncing chain.
-    latest_slot: Slot,
     /// Sync logger.
     log: slog::Logger,
 }
 
 impl SimpleSync {
+    /// Instantiate a `SimpleSync` instance, with no peers and an empty queue.
     pub fn new(beacon_chain: Arc<BeaconChain>, log: &slog::Logger) -> Self {
-        let state = beacon_chain.get_state();
         let sync_logger = log.new(o!("Service"=> "Sync"));
+
+        let queue_item_stale_time = Duration::from_secs(QUEUE_STALE_SECS);
+
+        let import_queue =
+            ImportQueue::new(beacon_chain.clone(), queue_item_stale_time, log.clone());
         SimpleSync {
             chain: beacon_chain.clone(),
             known_peers: HashMap::new(),
+            import_queue,
             state: SyncState::Idle,
-            network_id: beacon_chain.get_spec().network_id,
-            latest_finalized_epoch: state.finalized_epoch,
-            latest_slot: state.slot - 1, //TODO: Build latest block function into Beacon chain and correct this
             log: sync_logger,
         }
     }
 
-    /// Generates our current state in the form of a HELLO RPC message.
-    pub fn generate_hello(&self) -> HelloMessage {
-        let state = &self.chain.get_state();
-        //TODO: Paul to verify the logic of these fields.
-        HelloMessage {
-            network_id: self.network_id,
-            latest_finalized_root: state.finalized_root,
-            latest_finalized_epoch: state.finalized_epoch,
-            best_root: Hash256::zero(), //TODO: build correct value as a beacon chain function
-            best_slot: state.slot - 1,
+    /// Handle a `Goodbye` message from a peer.
+    ///
+    /// Removes the peer from `known_peers`.
+    pub fn on_goodbye(&mut self, peer_id: PeerId, reason: GoodbyeReason) {
+        info!(
+            self.log, "PeerGoodbye";
+            "peer" => format!("{:?}", peer_id),
+            "reason" => format!("{:?}", reason),
+        );
+
+        self.known_peers.remove(&peer_id);
+    }
+
+    /// Handle the connection of a new peer.
+    ///
+    /// Sends a `Hello` message to the peer.
+    pub fn on_connect(&self, peer_id: PeerId, network: &mut NetworkContext) {
+        info!(self.log, "PeerConnect"; "peer" => format!("{:?}", peer_id));
+
+        network.send_rpc_request(peer_id, RPCRequest::Hello(self.chain.hello_message()));
+    }
+
+    /// Handle a `Hello` request.
+    ///
+    /// Processes the `HelloMessage` from the remote peer and sends back our `Hello`.
+    pub fn on_hello_request(
+        &mut self,
+        peer_id: PeerId,
+        request_id: RequestId,
+        hello: HelloMessage,
+        network: &mut NetworkContext,
+    ) {
+        debug!(self.log, "HelloRequest"; "peer" => format!("{:?}", peer_id));
+
+        // Say hello back.
+        network.send_rpc_response(
+            peer_id.clone(),
+            request_id,
+            RPCResponse::Hello(self.chain.hello_message()),
+        );
+
+        self.process_hello(peer_id, hello, network);
+    }
+
+    /// Process a `Hello` response from a peer.
+    pub fn on_hello_response(
+        &mut self,
+        peer_id: PeerId,
+        hello: HelloMessage,
+        network: &mut NetworkContext,
+    ) {
+        debug!(self.log, "HelloResponse"; "peer" => format!("{:?}", peer_id));
+
+        // Process the hello message, without sending back another hello.
+        self.process_hello(peer_id, hello, network);
+    }
+
+    /// Returns a `PeerStatus` for some peer.
+    fn peer_status(&self, peer: PeerSyncInfo) -> PeerStatus {
+        let local = PeerSyncInfo::from(&self.chain);
+
+        if peer.has_different_network_id_to(local) {
+            return PeerStatus::DifferentNetworkId;
+        }
+
+        if local.has_higher_finalized_epoch_than(peer) {
+            let peer_finalized_slot = peer
+                .latest_finalized_epoch
+                .start_slot(self.chain.get_spec().slots_per_epoch);
+
+            let local_roots = self.chain.get_block_roots(peer_finalized_slot, 1, 0);
+
+            if let Ok(local_roots) = local_roots {
+                if let Some(local_root) = local_roots.get(0) {
+                    if *local_root != peer.latest_finalized_root {
+                        return PeerStatus::FinalizedEpochNotInChain;
+                    }
+                } else {
+                    error!(
+                        self.log,
+                        "Cannot get root for peer finalized slot.";
+                        "error" => "empty roots"
+                    );
+                }
+            } else {
+                error!(
+                    self.log,
+                    "Cannot get root for peer finalized slot.";
+                    "error" => format!("{:?}", local_roots)
+                );
+            }
+        }
+
+        if peer.has_higher_finalized_epoch_than(local) {
+            PeerStatus::HigherFinalizedEpoch
+        } else if peer.has_higher_best_slot_than(local) {
+            PeerStatus::HigherBestSlot
+        } else {
+            PeerStatus::NotInteresting
         }
     }
 
-    pub fn validate_peer(&mut self, peer_id: PeerId, hello_message: HelloMessage) -> bool {
-        // network id must match
-        if hello_message.network_id != self.network_id {
-            return false;
-        }
-        // compare latest epoch and finalized root to see if they exist in our chain
-        if hello_message.latest_finalized_epoch <= self.latest_finalized_epoch {
-            // ensure their finalized root is in our chain
-            // TODO: Get the finalized root at hello_message.latest_epoch and ensure they match
-            //if (hello_message.latest_finalized_root == self.chain.get_state() {
-            //    return false;
-            //    }
+    /// Process a `Hello` message, requesting new blocks if appropriate.
+    ///
+    /// Disconnects the peer if required.
+    fn process_hello(
+        &mut self,
+        peer_id: PeerId,
+        hello: HelloMessage,
+        network: &mut NetworkContext,
+    ) {
+        let spec = self.chain.get_spec();
+
+        let remote = PeerSyncInfo::from(hello);
+        let local = PeerSyncInfo::from(&self.chain);
+        let remote_status = self.peer_status(remote);
+
+        if remote_status.should_handshake() {
+            info!(self.log, "HandshakeSuccess"; "peer" => format!("{:?}", peer_id));
+            self.known_peers.insert(peer_id.clone(), remote);
+        } else {
+            info!(
+                self.log, "HandshakeFailure";
+                "peer" => format!("{:?}", peer_id),
+                "reason" => "network_id"
+            );
+            network.disconnect(peer_id.clone(), GoodbyeReason::IrreleventNetwork);
         }
 
-        // the client is valid, add it to our list of known_peers and request sync if required
-        // update peer list if peer already exists
-        let peer_info = PeerSyncInfo {
-            latest_finalized_root: hello_message.latest_finalized_root,
-            latest_finalized_epoch: hello_message.latest_finalized_epoch,
-            best_root: hello_message.best_root,
-            best_slot: hello_message.best_slot,
+        // If required, send additional requests.
+        match remote_status {
+            PeerStatus::HigherFinalizedEpoch => {
+                let start_slot = remote
+                    .latest_finalized_epoch
+                    .start_slot(spec.slots_per_epoch);
+                let required_slots = start_slot - local.best_slot;
+
+                self.request_block_roots(
+                    peer_id,
+                    BeaconBlockRootsRequest {
+                        start_slot,
+                        count: required_slots.into(),
+                    },
+                    network,
+                );
+            }
+            PeerStatus::HigherBestSlot => {
+                let required_slots = remote.best_slot - local.best_slot;
+
+                self.request_block_roots(
+                    peer_id,
+                    BeaconBlockRootsRequest {
+                        start_slot: local.best_slot + 1,
+                        count: required_slots.into(),
+                    },
+                    network,
+                );
+            }
+            PeerStatus::FinalizedEpochNotInChain => {}
+            PeerStatus::DifferentNetworkId => {}
+            PeerStatus::NotInteresting => {}
+        }
+    }
+
+    /// Handle a `BeaconBlockRoots` request from the peer.
+    pub fn on_beacon_block_roots_request(
+        &mut self,
+        peer_id: PeerId,
+        request_id: RequestId,
+        req: BeaconBlockRootsRequest,
+        network: &mut NetworkContext,
+    ) {
+        debug!(
+            self.log,
+            "BlockRootsRequest";
+            "peer" => format!("{:?}", peer_id),
+            "count" => req.count,
+        );
+
+        let roots = match self
+            .chain
+            .get_block_roots(req.start_slot, req.count as usize, 0)
+        {
+            Ok(roots) => roots,
+            Err(e) => {
+                // TODO: return RPC error.
+                warn!(
+                    self.log,
+                    "RPCRequest"; "peer" => format!("{:?}", peer_id),
+                    "req" => "BeaconBlockRoots",
+                    "error" => format!("{:?}", e)
+                );
+                return;
+            }
         };
 
-        debug!(self.log, "Handshake successful. Peer: {:?}", peer_id);
-        self.known_peers.insert(peer_id, peer_info);
+        let roots = roots
+            .iter()
+            .enumerate()
+            .map(|(i, &block_root)| BlockRootSlot {
+                slot: req.start_slot + Slot::from(i),
+                block_root,
+            })
+            .collect();
 
-        // set state to sync
-        if self.state == SyncState::Idle
-            && hello_message.best_slot > self.latest_slot + SLOT_IMPORT_TOLERANCE
-        {
-            self.state = SyncState::Downloading;
-            //TODO: Start requesting blocks from known peers. Ideally in batches
+        network.send_rpc_response(
+            peer_id,
+            request_id,
+            RPCResponse::BeaconBlockRoots(BeaconBlockRootsResponse { roots }),
+        )
+    }
+
+    /// Handle a `BeaconBlockRoots` response from the peer.
+    pub fn on_beacon_block_roots_response(
+        &mut self,
+        peer_id: PeerId,
+        res: BeaconBlockRootsResponse,
+        network: &mut NetworkContext,
+    ) {
+        debug!(
+            self.log,
+            "BlockRootsResponse";
+            "peer" => format!("{:?}", peer_id),
+            "count" => res.roots.len(),
+        );
+
+        if res.roots.is_empty() {
+            warn!(
+                self.log,
+                "Peer returned empty block roots response. PeerId: {:?}", peer_id
+            );
+            return;
         }
 
-        true
+        let new_root_index = self.import_queue.first_new_root(&res.roots);
+
+        // If a new block root is found, request it and all the headers following it.
+        //
+        // We make an assumption here that if we don't know a block then we don't know of all
+        // it's parents. This might not be the case if syncing becomes more sophisticated.
+        if let Some(i) = new_root_index {
+            let new = &res.roots[i];
+
+            self.request_block_headers(
+                peer_id,
+                BeaconBlockHeadersRequest {
+                    start_root: new.block_root,
+                    start_slot: new.slot,
+                    max_headers: (res.roots.len() - i) as u64,
+                    skip_slots: 0,
+                },
+                network,
+            )
+        }
+    }
+
+    /// Handle a `BeaconBlockHeaders` request from the peer.
+    pub fn on_beacon_block_headers_request(
+        &mut self,
+        peer_id: PeerId,
+        request_id: RequestId,
+        req: BeaconBlockHeadersRequest,
+        network: &mut NetworkContext,
+    ) {
+        debug!(
+            self.log,
+            "BlockHeadersRequest";
+            "peer" => format!("{:?}", peer_id),
+            "count" => req.max_headers,
+        );
+
+        let headers = match self.chain.get_block_headers(
+            req.start_slot,
+            req.max_headers as usize,
+            req.skip_slots as usize,
+        ) {
+            Ok(headers) => headers,
+            Err(e) => {
+                // TODO: return RPC error.
+                warn!(
+                    self.log,
+                    "RPCRequest"; "peer" => format!("{:?}", peer_id),
+                    "req" => "BeaconBlockHeaders",
+                    "error" => format!("{:?}", e)
+                );
+                return;
+            }
+        };
+
+        network.send_rpc_response(
+            peer_id,
+            request_id,
+            RPCResponse::BeaconBlockHeaders(BeaconBlockHeadersResponse { headers }),
+        )
+    }
+
+    /// Handle a `BeaconBlockHeaders` response from the peer.
+    pub fn on_beacon_block_headers_response(
+        &mut self,
+        peer_id: PeerId,
+        res: BeaconBlockHeadersResponse,
+        network: &mut NetworkContext,
+    ) {
+        debug!(
+            self.log,
+            "BlockHeadersResponse";
+            "peer" => format!("{:?}", peer_id),
+            "count" => res.headers.len(),
+        );
+
+        if res.headers.is_empty() {
+            warn!(
+                self.log,
+                "Peer returned empty block headers response. PeerId: {:?}", peer_id
+            );
+            return;
+        }
+
+        // Enqueue the headers, obtaining a list of the roots of the headers which were newly added
+        // to the queue.
+        let block_roots = self
+            .import_queue
+            .enqueue_headers(res.headers, peer_id.clone());
+
+        self.request_block_bodies(peer_id, BeaconBlockBodiesRequest { block_roots }, network);
+    }
+
+    /// Handle a `BeaconBlockBodies` request from the peer.
+    pub fn on_beacon_block_bodies_request(
+        &mut self,
+        peer_id: PeerId,
+        request_id: RequestId,
+        req: BeaconBlockBodiesRequest,
+        network: &mut NetworkContext,
+    ) {
+        debug!(
+            self.log,
+            "BlockBodiesRequest";
+            "peer" => format!("{:?}", peer_id),
+            "count" => req.block_roots.len(),
+        );
+
+        let block_bodies = match self.chain.get_block_bodies(&req.block_roots) {
+            Ok(bodies) => bodies,
+            Err(e) => {
+                // TODO: return RPC error.
+                warn!(
+                    self.log,
+                    "RPCRequest"; "peer" => format!("{:?}", peer_id),
+                    "req" => "BeaconBlockBodies",
+                    "error" => format!("{:?}", e)
+                );
+                return;
+            }
+        };
+
+        network.send_rpc_response(
+            peer_id,
+            request_id,
+            RPCResponse::BeaconBlockBodies(BeaconBlockBodiesResponse { block_bodies }),
+        )
+    }
+
+    /// Handle a `BeaconBlockBodies` response from the peer.
+    pub fn on_beacon_block_bodies_response(
+        &mut self,
+        peer_id: PeerId,
+        res: BeaconBlockBodiesResponse,
+        network: &mut NetworkContext,
+    ) {
+        debug!(
+            self.log,
+            "BlockBodiesResponse";
+            "peer" => format!("{:?}", peer_id),
+            "count" => res.block_bodies.len(),
+        );
+
+        self.import_queue
+            .enqueue_bodies(res.block_bodies, peer_id.clone());
+
+        // Clear out old entries
+        self.import_queue.remove_stale();
+
+        // Import blocks, if possible.
+        self.process_import_queue(network);
+    }
+
+    /// Process a gossip message declaring a new block.
+    pub fn on_block_gossip(
+        &mut self,
+        peer_id: PeerId,
+        msg: BlockGossip,
+        network: &mut NetworkContext,
+    ) {
+        debug!(
+            self.log,
+            "BlockGossip";
+            "peer" => format!("{:?}", peer_id),
+        );
+        // TODO: filter out messages that a prior to the finalized slot.
+        //
+        // TODO: if the block is a few more slots ahead, try to get all block roots from then until
+        // now.
+        //
+        // Note: only requests the new block -- will fail if we don't have its parents.
+        if self.import_queue.is_new_block(&msg.root.block_root) {
+            self.request_block_headers(
+                peer_id,
+                BeaconBlockHeadersRequest {
+                    start_root: msg.root.block_root,
+                    start_slot: msg.root.slot,
+                    max_headers: 1,
+                    skip_slots: 0,
+                },
+                network,
+            )
+        }
+    }
+
+    /// Process a gossip message declaring a new attestation.
+    ///
+    /// Not currently implemented.
+    pub fn on_attestation_gossip(
+        &mut self,
+        peer_id: PeerId,
+        msg: AttestationGossip,
+        _network: &mut NetworkContext,
+    ) {
+        debug!(
+            self.log,
+            "AttestationGossip";
+            "peer" => format!("{:?}", peer_id),
+        );
+
+        // Awaiting a proper operations pool before we can import attestations.
+        //
+        // https://github.com/sigp/lighthouse/issues/281
+        match self.chain.process_attestation(msg.attestation) {
+            Ok(_) => panic!("Impossible, method not implemented."),
+            Err(_) => error!(self.log, "Attestation processing not implemented!"),
+        }
+    }
+
+    /// Iterate through the `import_queue` and process any complete blocks.
+    ///
+    /// If a block is successfully processed it is removed from the queue, otherwise it remains in
+    /// the queue.
+    pub fn process_import_queue(&mut self, network: &mut NetworkContext) {
+        let mut successful = 0;
+        let mut invalid = 0;
+        let mut errored = 0;
+
+        // Loop through all of the complete blocks in the queue.
+        for (block_root, block, sender) in self.import_queue.complete_blocks() {
+            match self.chain.process_block(block) {
+                Ok(outcome) => {
+                    if outcome.is_invalid() {
+                        invalid += 1;
+                        warn!(
+                            self.log,
+                            "InvalidBlock";
+                            "sender_peer_id" => format!("{:?}", sender),
+                            "reason" => format!("{:?}", outcome),
+                        );
+                        network.disconnect(sender, GoodbyeReason::Fault);
+                    }
+
+                    // If this results to true, the item will be removed from the queue.
+                    if outcome.sucessfully_processed() {
+                        successful += 1;
+                        self.import_queue.remove(block_root);
+                    }
+                }
+                Err(e) => {
+                    errored += 1;
+                    error!(self.log, "BlockProcessingError"; "error" => format!("{:?}", e));
+                }
+            }
+        }
+
+        if successful > 0 {
+            info!(self.log, "Imported {} blocks", successful)
+        }
+        if invalid > 0 {
+            warn!(self.log, "Rejected {} invalid blocks", invalid)
+        }
+        if errored > 0 {
+            warn!(self.log, "Failed to process {} blocks", errored)
+        }
+    }
+
+    /// Request some `BeaconBlockRoots` from the remote peer.
+    fn request_block_roots(
+        &mut self,
+        peer_id: PeerId,
+        req: BeaconBlockRootsRequest,
+        network: &mut NetworkContext,
+    ) {
+        // Potentially set state to sync.
+        if self.state == SyncState::Idle && req.count > SLOT_IMPORT_TOLERANCE {
+            debug!(self.log, "Entering downloading sync state.");
+            self.state = SyncState::Downloading;
+        }
+
+        debug!(
+            self.log,
+            "RPCRequest(BeaconBlockRoots)";
+            "count" => req.count,
+            "peer" => format!("{:?}", peer_id)
+        );
+
+        // TODO: handle count > max count.
+        network.send_rpc_request(peer_id.clone(), RPCRequest::BeaconBlockRoots(req));
+    }
+
+    /// Request some `BeaconBlockHeaders` from the remote peer.
+    fn request_block_headers(
+        &mut self,
+        peer_id: PeerId,
+        req: BeaconBlockHeadersRequest,
+        network: &mut NetworkContext,
+    ) {
+        debug!(
+            self.log,
+            "RPCRequest(BeaconBlockHeaders)";
+            "max_headers" => req.max_headers,
+            "peer" => format!("{:?}", peer_id)
+        );
+
+        network.send_rpc_request(peer_id.clone(), RPCRequest::BeaconBlockHeaders(req));
+    }
+
+    /// Request some `BeaconBlockBodies` from the remote peer.
+    fn request_block_bodies(
+        &mut self,
+        peer_id: PeerId,
+        req: BeaconBlockBodiesRequest,
+        network: &mut NetworkContext,
+    ) {
+        debug!(
+            self.log,
+            "RPCRequest(BeaconBlockBodies)";
+            "count" => req.block_roots.len(),
+            "peer" => format!("{:?}", peer_id)
+        );
+
+        network.send_rpc_request(peer_id.clone(), RPCRequest::BeaconBlockBodies(req));
+    }
+
+    /// Generates our current state in the form of a HELLO RPC message.
+    pub fn generate_hello(&self) -> HelloMessage {
+        self.chain.hello_message()
     }
 }
diff --git a/beacon_node/network/tests/tests.rs b/beacon_node/network/tests/tests.rs
new file mode 100644
index 000000000..9cead1b55
--- /dev/null
+++ b/beacon_node/network/tests/tests.rs
@@ -0,0 +1,570 @@
+use crossbeam_channel::{unbounded, Receiver, RecvTimeoutError, Sender};
+use eth2_libp2p::rpc::methods::*;
+use eth2_libp2p::rpc::{RPCMethod, RPCRequest, RPCResponse, RequestId};
+use eth2_libp2p::{PeerId, RPCEvent};
+use network::beacon_chain::BeaconChain as NetworkBeaconChain;
+use network::message_handler::{HandlerMessage, MessageHandler};
+use network::service::{NetworkMessage, OutgoingMessage};
+use sloggers::terminal::{Destination, TerminalLoggerBuilder};
+use sloggers::types::Severity;
+use sloggers::Build;
+use std::time::Duration;
+use test_harness::BeaconChainHarness;
+use tokio::runtime::TaskExecutor;
+use types::{test_utils::TestingBeaconStateBuilder, *};
+
+pub struct SyncNode {
+    pub id: usize,
+    sender: Sender<HandlerMessage>,
+    receiver: Receiver<NetworkMessage>,
+    peer_id: PeerId,
+    harness: BeaconChainHarness,
+}
+
+impl SyncNode {
+    fn from_beacon_state_builder(
+        id: usize,
+        executor: &TaskExecutor,
+        state_builder: TestingBeaconStateBuilder,
+        spec: &ChainSpec,
+        logger: slog::Logger,
+    ) -> Self {
+        let harness = BeaconChainHarness::from_beacon_state_builder(state_builder, spec.clone());
+
+        let (network_sender, network_receiver) = unbounded();
+        let message_handler_sender = MessageHandler::spawn(
+            harness.beacon_chain.clone(),
+            network_sender,
+            executor,
+            logger,
+        )
+        .unwrap();
+
+        Self {
+            id,
+            sender: message_handler_sender,
+            receiver: network_receiver,
+            peer_id: PeerId::random(),
+            harness,
+        }
+    }
+
+    fn increment_beacon_chain_slot(&mut self) {
+        self.harness.increment_beacon_chain_slot();
+    }
+
+    fn send(&self, message: HandlerMessage) {
+        self.sender.send(message).unwrap();
+    }
+
+    fn recv(&self) -> Result<NetworkMessage, RecvTimeoutError> {
+        self.receiver.recv_timeout(Duration::from_millis(500))
+    }
+
+    fn hello_message(&self) -> HelloMessage {
+        self.harness.beacon_chain.hello_message()
+    }
+
+    pub fn connect_to(&mut self, node: &SyncNode) {
+        let message = HandlerMessage::PeerDialed(self.peer_id.clone());
+        node.send(message);
+    }
+
+    /// Reads the receive queue from one node and passes the message to the other. Also returns a
+    /// copy of the message.
+    ///
+    /// self -----> node
+    ///        |
+    ///        us
+    ///
+    /// Named after the unix `tee` command.
+    fn tee(&mut self, node: &SyncNode) -> NetworkMessage {
+        let network_message = self.recv().expect("Timeout on tee");
+
+        let handler_message = match network_message.clone() {
+            NetworkMessage::Send(_to_peer_id, OutgoingMessage::RPC(event)) => {
+                HandlerMessage::RPC(self.peer_id.clone(), event)
+            }
+            _ => panic!("tee cannot parse {:?}", network_message),
+        };
+
+        node.send(handler_message);
+
+        network_message
+    }
+
+    fn tee_hello_request(&mut self, node: &SyncNode) -> HelloMessage {
+        let request = self.tee_rpc_request(node);
+
+        match request {
+            RPCRequest::Hello(message) => message,
+            _ => panic!("tee_hello_request got: {:?}", request),
+        }
+    }
+
+    fn tee_hello_response(&mut self, node: &SyncNode) -> HelloMessage {
+        let response = self.tee_rpc_response(node);
+
+        match response {
+            RPCResponse::Hello(message) => message,
+            _ => panic!("tee_hello_response got: {:?}", response),
+        }
+    }
+
+    fn tee_block_root_request(&mut self, node: &SyncNode) -> BeaconBlockRootsRequest {
+        let msg = self.tee_rpc_request(node);
+
+        match msg {
+            RPCRequest::BeaconBlockRoots(data) => data,
+            _ => panic!("tee_block_root_request got: {:?}", msg),
+        }
+    }
+
+    fn tee_block_root_response(&mut self, node: &SyncNode) -> BeaconBlockRootsResponse {
+        let msg = self.tee_rpc_response(node);
+
+        match msg {
+            RPCResponse::BeaconBlockRoots(data) => data,
+            _ => panic!("tee_block_root_response got: {:?}", msg),
+        }
+    }
+
+    fn tee_block_header_request(&mut self, node: &SyncNode) -> BeaconBlockHeadersRequest {
+        let msg = self.tee_rpc_request(node);
+
+        match msg {
+            RPCRequest::BeaconBlockHeaders(data) => data,
+            _ => panic!("tee_block_header_request got: {:?}", msg),
+        }
+    }
+
+    fn tee_block_header_response(&mut self, node: &SyncNode) -> BeaconBlockHeadersResponse {
+        let msg = self.tee_rpc_response(node);
+
+        match msg {
+            RPCResponse::BeaconBlockHeaders(data) => data,
+            _ => panic!("tee_block_header_response got: {:?}", msg),
+        }
+    }
+
+    fn tee_block_body_request(&mut self, node: &SyncNode) -> BeaconBlockBodiesRequest {
+        let msg = self.tee_rpc_request(node);
+
+        match msg {
+            RPCRequest::BeaconBlockBodies(data) => data,
+            _ => panic!("tee_block_body_request got: {:?}", msg),
+        }
+    }
+
+    fn tee_block_body_response(&mut self, node: &SyncNode) -> BeaconBlockBodiesResponse {
+        let msg = self.tee_rpc_response(node);
+
+        match msg {
+            RPCResponse::BeaconBlockBodies(data) => data,
+            _ => panic!("tee_block_body_response got: {:?}", msg),
+        }
+    }
+
+    fn tee_rpc_request(&mut self, node: &SyncNode) -> RPCRequest {
+        let network_message = self.tee(node);
+
+        match network_message {
+            NetworkMessage::Send(
+                _peer_id,
+                OutgoingMessage::RPC(RPCEvent::Request {
+                    id: _,
+                    method_id: _,
+                    body,
+                }),
+            ) => body,
+            _ => panic!("tee_rpc_request failed! got {:?}", network_message),
+        }
+    }
+
+    fn tee_rpc_response(&mut self, node: &SyncNode) -> RPCResponse {
+        let network_message = self.tee(node);
+
+        match network_message {
+            NetworkMessage::Send(
+                _peer_id,
+                OutgoingMessage::RPC(RPCEvent::Response {
+                    id: _,
+                    method_id: _,
+                    result,
+                }),
+            ) => result,
+            _ => panic!("tee_rpc_response failed! got {:?}", network_message),
+        }
+    }
+
+    pub fn get_block_root_request(&self) -> BeaconBlockRootsRequest {
+        let request = self.recv_rpc_request().expect("No block root request");
+
+        match request {
+            RPCRequest::BeaconBlockRoots(request) => request,
+            _ => panic!("Did not get block root request"),
+        }
+    }
+
+    pub fn get_block_headers_request(&self) -> BeaconBlockHeadersRequest {
+        let request = self.recv_rpc_request().expect("No block headers request");
+
+        match request {
+            RPCRequest::BeaconBlockHeaders(request) => request,
+            _ => panic!("Did not get block headers request"),
+        }
+    }
+
+    pub fn get_block_bodies_request(&self) -> BeaconBlockBodiesRequest {
+        let request = self.recv_rpc_request().expect("No block bodies request");
+
+        match request {
+            RPCRequest::BeaconBlockBodies(request) => request,
+            _ => panic!("Did not get block bodies request"),
+        }
+    }
+
+    fn _recv_rpc_response(&self) -> Result<RPCResponse, RecvTimeoutError> {
+        let network_message = self.recv()?;
+        Ok(match network_message {
+            NetworkMessage::Send(
+                _peer_id,
+                OutgoingMessage::RPC(RPCEvent::Response {
+                    id: _,
+                    method_id: _,
+                    result,
+                }),
+            ) => result,
+            _ => panic!("get_rpc_response failed! got {:?}", network_message),
+        })
+    }
+
+    fn recv_rpc_request(&self) -> Result<RPCRequest, RecvTimeoutError> {
+        let network_message = self.recv()?;
+        Ok(match network_message {
+            NetworkMessage::Send(
+                _peer_id,
+                OutgoingMessage::RPC(RPCEvent::Request {
+                    id: _,
+                    method_id: _,
+                    body,
+                }),
+            ) => body,
+            _ => panic!("get_rpc_request failed! got {:?}", network_message),
+        })
+    }
+}
+
+fn get_logger() -> slog::Logger {
+    let mut builder = TerminalLoggerBuilder::new();
+    builder.level(Severity::Debug);
+    builder.destination(Destination::Stderr);
+    builder.build().unwrap()
+}
+
+pub struct SyncMaster {
+    harness: BeaconChainHarness,
+    peer_id: PeerId,
+    response_ids: Vec<RequestId>,
+}
+
+impl SyncMaster {
+    fn from_beacon_state_builder(
+        state_builder: TestingBeaconStateBuilder,
+        node_count: usize,
+        spec: &ChainSpec,
+    ) -> Self {
+        let harness = BeaconChainHarness::from_beacon_state_builder(state_builder, spec.clone());
+        let peer_id = PeerId::random();
+        let response_ids = vec![RequestId::from(0); node_count];
+
+        Self {
+            harness,
+            peer_id,
+            response_ids,
+        }
+    }
+
+    pub fn response_id(&mut self, node: &SyncNode) -> RequestId {
+        let id = self.response_ids[node.id].clone();
+        self.response_ids[node.id].increment();
+        id
+    }
+
+    pub fn do_hello_with(&mut self, node: &SyncNode) {
+        let message = HandlerMessage::PeerDialed(self.peer_id.clone());
+        node.send(message);
+
+        let request = node.recv_rpc_request().expect("No hello response");
+
+        match request {
+            RPCRequest::Hello(_hello) => {
+                let hello = self.harness.beacon_chain.hello_message();
+                let response = self.rpc_response(node, RPCResponse::Hello(hello));
+                node.send(response);
+            }
+            _ => panic!("Got message other than hello from node."),
+        }
+    }
+
+    pub fn respond_to_block_roots_request(
+        &mut self,
+        node: &SyncNode,
+        request: BeaconBlockRootsRequest,
+    ) {
+        let roots = self
+            .harness
+            .beacon_chain
+            .get_block_roots(request.start_slot, request.count as usize, 0)
+            .expect("Beacon chain did not give block roots")
+            .iter()
+            .enumerate()
+            .map(|(i, root)| BlockRootSlot {
+                block_root: *root,
+                slot: Slot::from(i) + request.start_slot,
+            })
+            .collect();
+
+        let response = RPCResponse::BeaconBlockRoots(BeaconBlockRootsResponse { roots });
+        self.send_rpc_response(node, response)
+    }
+
+    pub fn respond_to_block_headers_request(
+        &mut self,
+        node: &SyncNode,
+        request: BeaconBlockHeadersRequest,
+    ) {
+        let roots = self
+            .harness
+            .beacon_chain
+            .get_block_roots(
+                request.start_slot,
+                request.max_headers as usize,
+                request.skip_slots as usize,
+            )
+            .expect("Beacon chain did not give blocks");
+
+        if roots.is_empty() {
+            panic!("Roots was empty when trying to get headers.")
+        }
+
+        assert_eq!(
+            roots[0], request.start_root,
+            "Got the wrong start root when getting headers"
+        );
+
+        let headers: Vec<BeaconBlockHeader> = roots
+            .iter()
+            .map(|root| {
+                let block = self
+                    .harness
+                    .beacon_chain
+                    .get_block(root)
+                    .expect("Failed to load block")
+                    .expect("Block did not exist");
+                block.block_header()
+            })
+            .collect();
+
+        let response = RPCResponse::BeaconBlockHeaders(BeaconBlockHeadersResponse { headers });
+        self.send_rpc_response(node, response)
+    }
+
+    pub fn respond_to_block_bodies_request(
+        &mut self,
+        node: &SyncNode,
+        request: BeaconBlockBodiesRequest,
+    ) {
+        let block_bodies: Vec<BeaconBlockBody> = request
+            .block_roots
+            .iter()
+            .map(|root| {
+                let block = self
+                    .harness
+                    .beacon_chain
+                    .get_block(root)
+                    .expect("Failed to load block")
+                    .expect("Block did not exist");
+                block.body
+            })
+            .collect();
+
+        let response = RPCResponse::BeaconBlockBodies(BeaconBlockBodiesResponse { block_bodies });
+        self.send_rpc_response(node, response)
+    }
+
+    fn send_rpc_response(&mut self, node: &SyncNode, rpc_response: RPCResponse) {
+        node.send(self.rpc_response(node, rpc_response));
+    }
+
+    fn rpc_response(&mut self, node: &SyncNode, rpc_response: RPCResponse) -> HandlerMessage {
+        HandlerMessage::RPC(
+            self.peer_id.clone(),
+            RPCEvent::Response {
+                id: self.response_id(node),
+                method_id: RPCMethod::Hello.into(),
+                result: rpc_response,
+            },
+        )
+    }
+}
+
+fn test_setup(
+    state_builder: TestingBeaconStateBuilder,
+    node_count: usize,
+    spec: &ChainSpec,
+    logger: slog::Logger,
+) -> (tokio::runtime::Runtime, SyncMaster, Vec<SyncNode>) {
+    let runtime = tokio::runtime::Runtime::new().unwrap();
+
+    let mut nodes = Vec::with_capacity(node_count);
+    for id in 0..node_count {
+        let node = SyncNode::from_beacon_state_builder(
+            id,
+            &runtime.executor(),
+            state_builder.clone(),
+            &spec,
+            logger.clone(),
+        );
+
+        nodes.push(node);
+    }
+
+    let master = SyncMaster::from_beacon_state_builder(state_builder, node_count, &spec);
+
+    (runtime, master, nodes)
+}
+
+pub fn build_blocks(blocks: usize, master: &mut SyncMaster, nodes: &mut Vec<SyncNode>) {
+    for _ in 0..blocks {
+        master.harness.advance_chain_with_block();
+        for i in 0..nodes.len() {
+            nodes[i].increment_beacon_chain_slot();
+        }
+    }
+    master.harness.run_fork_choice();
+
+    for i in 0..nodes.len() {
+        nodes[i].harness.run_fork_choice();
+    }
+}
+
+#[test]
+#[ignore]
+fn sync_node_with_master() {
+    let logger = get_logger();
+    let spec = ChainSpec::few_validators();
+    let validator_count = 8;
+    let node_count = 1;
+
+    let state_builder =
+        TestingBeaconStateBuilder::from_default_keypairs_file_if_exists(validator_count, &spec);
+
+    let (runtime, mut master, mut nodes) =
+        test_setup(state_builder, node_count, &spec, logger.clone());
+
+    let original_node_slot = nodes[0].hello_message().best_slot;
+
+    build_blocks(2, &mut master, &mut nodes);
+
+    master.do_hello_with(&nodes[0]);
+
+    let roots_request = nodes[0].get_block_root_request();
+    assert_eq!(roots_request.start_slot, original_node_slot + 1);
+    assert_eq!(roots_request.count, 2);
+
+    master.respond_to_block_roots_request(&nodes[0], roots_request);
+
+    let headers_request = nodes[0].get_block_headers_request();
+    assert_eq!(headers_request.start_slot, original_node_slot + 1);
+    assert_eq!(headers_request.max_headers, 2);
+    assert_eq!(headers_request.skip_slots, 0);
+
+    master.respond_to_block_headers_request(&nodes[0], headers_request);
+
+    let bodies_request = nodes[0].get_block_bodies_request();
+    assert_eq!(bodies_request.block_roots.len(), 2);
+
+    master.respond_to_block_bodies_request(&nodes[0], bodies_request);
+
+    std::thread::sleep(Duration::from_millis(10000));
+    runtime.shutdown_now();
+}
+
+#[test]
+#[ignore]
+fn sync_two_nodes() {
+    let logger = get_logger();
+    let spec = ChainSpec::few_validators();
+    let validator_count = 8;
+    let node_count = 2;
+
+    let state_builder =
+        TestingBeaconStateBuilder::from_default_keypairs_file_if_exists(validator_count, &spec);
+
+    let (runtime, _master, mut nodes) =
+        test_setup(state_builder, node_count, &spec, logger.clone());
+
+    // let original_node_slot = nodes[0].hello_message().best_slot;
+    let mut node_a = nodes.remove(0);
+    let mut node_b = nodes.remove(0);
+
+    let blocks = 2;
+
+    // Node A builds out a longer, better chain.
+    for _ in 0..blocks {
+        // Node A should build a block.
+        node_a.harness.advance_chain_with_block();
+        // Node B should just increment it's slot without a block.
+        node_b.harness.increment_beacon_chain_slot();
+    }
+    node_a.harness.run_fork_choice();
+
+    // A connects to B.
+    node_a.connect_to(&node_b);
+
+    // B says hello to A.
+    node_b.tee_hello_request(&node_a);
+    // A says hello back.
+    node_a.tee_hello_response(&node_b);
+
+    // B requests block roots from A.
+    node_b.tee_block_root_request(&node_a);
+    // A provides block roots to A.
+    node_a.tee_block_root_response(&node_b);
+
+    // B requests block headers from A.
+    node_b.tee_block_header_request(&node_a);
+    // A provides block headers to B.
+    node_a.tee_block_header_response(&node_b);
+
+    // B requests block bodies from A.
+    node_b.tee_block_body_request(&node_a);
+    // A provides block bodies to B.
+    node_a.tee_block_body_response(&node_b);
+
+    std::thread::sleep(Duration::from_secs(10));
+
+    node_b.harness.run_fork_choice();
+
+    let node_a_chain = node_a
+        .harness
+        .beacon_chain
+        .chain_dump()
+        .expect("Can't dump node a chain");
+
+    let node_b_chain = node_b
+        .harness
+        .beacon_chain
+        .chain_dump()
+        .expect("Can't dump node b chain");
+
+    assert_eq!(
+        node_a_chain.len(),
+        node_b_chain.len(),
+        "Chains should be equal length"
+    );
+    assert_eq!(node_a_chain, node_b_chain, "Chains should be identical");
+
+    runtime.shutdown_now();
+}
diff --git a/eth2/types/src/beacon_block_header.rs b/eth2/types/src/beacon_block_header.rs
index 3d8b08cc8..f4bee27e1 100644
--- a/eth2/types/src/beacon_block_header.rs
+++ b/eth2/types/src/beacon_block_header.rs
@@ -37,6 +37,19 @@ impl BeaconBlockHeader {
     pub fn canonical_root(&self) -> Hash256 {
         Hash256::from_slice(&self.hash_tree_root()[..])
     }
+
+    /// Given a `body`, consumes `self` and returns a complete `BeaconBlock`.
+    ///
+    /// Spec v0.5.0
+    pub fn into_block(self, body: BeaconBlockBody) -> BeaconBlock {
+        BeaconBlock {
+            slot: self.slot,
+            previous_block_root: self.previous_block_root,
+            state_root: self.state_root,
+            body,
+            signature: self.signature,
+        }
+    }
 }
 
 #[cfg(test)]
diff --git a/eth2/types/src/test_utils/testing_beacon_state_builder.rs b/eth2/types/src/test_utils/testing_beacon_state_builder.rs
index 6945769aa..473cd4166 100644
--- a/eth2/types/src/test_utils/testing_beacon_state_builder.rs
+++ b/eth2/types/src/test_utils/testing_beacon_state_builder.rs
@@ -23,6 +23,7 @@ pub fn keypairs_path() -> PathBuf {
 /// Builds a beacon state to be used for testing purposes.
 ///
 /// This struct should **never be used for production purposes.**
+#[derive(Clone)]
 pub struct TestingBeaconStateBuilder {
     state: BeaconState,
     keypairs: Vec<Keypair>,