diff --git a/beacon_node/beacon_chain/src/attestation_aggregation.rs b/beacon_node/beacon_chain/src/attestation_aggregation.rs
new file mode 100644
index 0000000000..4cac5056cb
--- /dev/null
+++ b/beacon_node/beacon_chain/src/attestation_aggregation.rs
@@ -0,0 +1,129 @@
+use std::collections::{HashMap, HashSet};
+use types::{
+    AggregateSignature, Attestation, AttestationData, BeaconState, Bitfield, ChainSpec, Signature,
+};
+
+const PHASE_0_CUSTODY_BIT: bool = false;
+
+pub struct AttestationAggregator {
+    store: HashMap<Vec<u8>, Attestation>,
+}
+
+pub enum ProcessOutcome {
+    AggregationNotRequired,
+    Aggregated,
+    NewAttestationCreated,
+}
+
+pub enum ProcessError {
+    BadValidatorIndex,
+    BadSignature,
+}
+
+impl AttestationAggregator {
+    pub fn new() -> Self {
+        Self {
+            store: HashMap::new(),
+        }
+    }
+
+    pub fn process_free_attestation(
+        &mut self,
+        state: &BeaconState,
+        attestation_data: &AttestationData,
+        signature: &Signature,
+        validator_index: u64,
+    ) -> Result<ProcessOutcome, ProcessError> {
+        let validator_index = validator_index as usize;
+
+        let signable_message = attestation_data.signable_message(PHASE_0_CUSTODY_BIT);
+        let validator_pubkey = &state
+            .validator_registry
+            .get(validator_index)
+            .ok_or_else(|| ProcessError::BadValidatorIndex)?
+            .pubkey;
+
+        if !signature.verify(&signable_message, &validator_pubkey) {
+            return Err(ProcessError::BadSignature);
+        }
+
+        if let Some(existing_attestation) = self.store.get(&signable_message) {
+            if let Some(updated_attestation) =
+                aggregate_attestation(existing_attestation, signature, validator_index)
+            {
+                self.store.insert(signable_message, updated_attestation);
+                Ok(ProcessOutcome::Aggregated)
+            } else {
+                Ok(ProcessOutcome::AggregationNotRequired)
+            }
+        } else {
+            let mut aggregate_signature = AggregateSignature::new();
+            aggregate_signature.add(signature);
+            let mut aggregation_bitfield = Bitfield::new();
+            aggregation_bitfield.set(validator_index, true);
+            let new_attestation = Attestation {
+                data: attestation_data.clone(),
+                aggregation_bitfield,
+                custody_bitfield: Bitfield::new(),
+                aggregate_signature,
+            };
+            self.store.insert(signable_message, new_attestation);
+            Ok(ProcessOutcome::NewAttestationCreated)
+        }
+    }
+
+    /// Returns all known attestations which are:
+    ///
+    /// a) valid for the given state
+    /// b) not already in `state.latest_attestations`.
+    pub fn get_attestations_for_state(
+        &self,
+        state: &BeaconState,
+        spec: &ChainSpec,
+    ) -> Vec<Attestation> {
+        let mut known_attestation_data: HashSet<AttestationData> = HashSet::new();
+
+        state.latest_attestations.iter().for_each(|attestation| {
+            known_attestation_data.insert(attestation.data.clone());
+        });
+
+        self.store
+            .values()
+            .filter_map(|attestation| {
+                if state.validate_attestation(attestation, spec).is_ok()
+                    && !known_attestation_data.contains(&attestation.data)
+                {
+                    Some(attestation.clone())
+                } else {
+                    None
+                }
+            })
+            .collect()
+    }
+}
+
+fn aggregate_attestation(
+    existing_attestation: &Attestation,
+    signature: &Signature,
+    validator_index: usize,
+) -> Option<Attestation> {
+    let already_signed = existing_attestation
+        .aggregation_bitfield
+        .get(validator_index)
+        .unwrap_or(false);
+
+    if already_signed {
+        None
+    } else {
+        let mut aggregation_bitfield = existing_attestation.aggregation_bitfield.clone();
+        aggregation_bitfield.set(validator_index, true);
+        let mut aggregate_signature = existing_attestation.aggregate_signature.clone();
+        aggregate_signature.add(&signature);
+
+        Some(Attestation {
+            aggregation_bitfield,
+            aggregate_signature,
+            ..existing_attestation.clone()
+        })
+    }
+}
diff --git a/beacon_node/beacon_chain/src/lib.rs b/beacon_node/beacon_chain/src/lib.rs
index bdb041b130..c4da9099f1 100644
--- a/beacon_node/beacon_chain/src/lib.rs
+++ b/beacon_node/beacon_chain/src/lib.rs
@@ -1,3 +1,4 @@
+mod attestation_aggregation;
 mod attestation_production;
 mod attestation_targets;
 mod block_graph;
@@ -13,6 +14,7 @@ mod state_transition;
 
 use self::attestation_targets::AttestationTargets;
 use self::block_graph::BlockGraph;
+use attestation_aggregation::AttestationAggregator;
 use db::{
     stores::{BeaconBlockStore, BeaconStateStore},
     ClientDB, DBError,
@@ -73,6 +75,7 @@ pub struct BeaconChain<T: ClientDB + Sized, U: SlotClock> {
     pub state_store: Arc<BeaconStateStore<T>>,
     pub slot_clock: U,
     pub block_graph: BlockGraph,
+    pub attestation_aggregator: RwLock<AttestationAggregator>,
     canonical_head: RwLock<CheckPoint>,
     finalized_head: RwLock<CheckPoint>,
     justified_head: RwLock<CheckPoint>,
@@ -124,6 +127,7 @@ where
             genesis_state.clone(),
             state_root.clone(),
         ));
+        let attestation_aggregator = RwLock::new(AttestationAggregator::new());
 
         let latest_attestation_targets = RwLock::new(AttestationTargets::new());
 
@@ -132,6 +136,7 @@ where
             state_store,
             slot_clock,
             block_graph,
+            attestation_aggregator,
             justified_head,
             finalized_head,
             canonical_head,
diff --git a/eth2/types/src/attestation_data/mod.rs b/eth2/types/src/attestation_data/mod.rs
index f22b4007f1..28504127ee 100644
--- a/eth2/types/src/attestation_data/mod.rs
+++ b/eth2/types/src/attestation_data/mod.rs
@@ -3,6 +3,7 @@ use crate::test_utils::TestRandom;
 use rand::RngCore;
 use serde_derive::Serialize;
 use ssz::{hash, Decodable, DecodeError, Encodable, SszStream, TreeHash};
+use std::hash::Hash;
 
 mod signing;
 
@@ -17,7 +18,7 @@ pub const SSZ_ATTESTION_DATA_LENGTH: usize = {
     32 // justified_block_root
 };
 
-#[derive(Debug, Clone, PartialEq, Default, Serialize)]
+#[derive(Debug, Clone, PartialEq, Default, Serialize, Hash)]
 pub struct AttestationData {
     pub slot: u64,
     pub shard: u64,
@@ -29,6 +30,8 @@ pub struct AttestationData {
     pub justified_block_root: Hash256,
 }
 
+impl Eq for AttestationData {}
+
 impl AttestationData {
     pub fn zero() -> Self {
         Self {