diff --git a/beacon_chain/attestation_aggregation.nim b/beacon_chain/attestation_aggregation.nim
index 0c0f1b7b5..584e47e9e 100644
--- a/beacon_chain/attestation_aggregation.nim
+++ b/beacon_chain/attestation_aggregation.nim
@@ -5,20 +5,12 @@
 #   * Apache v2 license (license terms in the root directory or at http://www.apache.org/licenses/LICENSE-2.0).
 # at your option. This file may not be copied, modified, or distributed except according to those terms.
 
-# Have an an aggregated aggregation ready for broadcast at
-# SECONDS_PER_SLOT * 2 / 3, i.e. 2/3 through relevant slot
-# intervals.
-#
 # The other part is arguably part of attestation pool -- the validation's
 # something that should be happing on receipt, not aggregation per se. In
 # that part, check that messages conform -- so, check for each type
 # https://github.com/ethereum/eth2.0-specs/blob/v0.10.1/specs/phase0/p2p-interface.md#topics-and-messages
 # specifies. So by the time this calls attestation pool, all validation's
 # already done.
-#
-# Finally, some of the filtering's libp2p stuff. Consistency checks between
-# topic/message types and GOSSIP_MAX_SIZE -- mostly doesn't belong here, so
-# while TODO, isn't TODO for this module.
 
 import
   options,
@@ -26,11 +18,7 @@ import
     state_transition_block],
   ./attestation_pool, ./beacon_node_types, ./ssz
 
-# TODO gossipsub validation lives somewhere, maybe here
-# TODO add tests, especially for validation
-# https://github.com/status-im/nim-beacon-chain/issues/122#issuecomment-562479965
-
-# https://github.com/ethereum/eth2.0-specs/blob/v0.10.1/specs/phase0/validator.md#aggregation-selection
+# https://github.com/ethereum/eth2.0-specs/blob/v0.11.1/specs/phase0/validator.md#aggregation-selection
 func is_aggregator(state: BeaconState, slot: Slot, index: uint64,
     slot_signature: ValidatorSig): bool =
   # TODO index is a CommitteeIndex, aka uint64
@@ -43,34 +31,49 @@ func is_aggregator(state: BeaconState, slot: Slot, index: uint64,
 
 proc aggregate_attestations*(
     pool: AttestationPool, state: BeaconState, index: uint64,
-    privkey: ValidatorPrivKey): Option[AggregateAndProof] =
+    privkey: ValidatorPrivKey, trailing_distance: uint64): Option[AggregateAndProof] =
   # TODO alias CommitteeIndex to actual type then convert various uint64's here
 
-  let
-    slot = state.slot - 2
-    slot_signature = get_slot_signature(state.fork, slot, privkey)
+  doAssert state.slot >= trailing_distance
+
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.11.1/specs/phase0/p2p-interface.md#configuration
+  doAssert trailing_distance <= ATTESTATION_PROPAGATION_SLOT_RANGE
+
+  let
+    slot = state.slot - trailing_distance
+    slot_signature = get_slot_signature(
+      state.fork, state.genesis_validators_root, slot, privkey)
 
-  if slot < 0:
-    return none(AggregateAndProof)
   doAssert slot + ATTESTATION_PROPAGATION_SLOT_RANGE >= state.slot
   doAssert state.slot >= slot
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.10.1/specs/phase0/validator.md#aggregation-selection
+  # TODO performance issue for future, via get_active_validator_indices(...)
+  doAssert index < get_committee_count_at_slot(state, slot)
+
+  # TODO for testing purposes, refactor this into the condition check
+  # and just calculation
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.11.1/specs/phase0/validator.md#aggregation-selection
   if not is_aggregator(state, slot, index, slot_signature):
     return none(AggregateAndProof)
 
-  let attestation_data =
-    makeAttestationData(state, slot, index, get_block_root_at_slot(state, slot))
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.11.1/specs/phase0/validator.md#attestation-data
+  # describes how to construct an attestation, which applies for makeAttestationData(...)
+  # TODO this won't actually match anything
+  let attestation_data = AttestationData(
+    slot: slot,
+    index: index,
+    beacon_block_root: get_block_root_at_slot(state, slot))
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.10.1/specs/phase0/validator.md#construct-aggregate
-  for attestation in getAttestationsForBlock(pool, state, slot):
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.11.1/specs/phase0/validator.md#construct-aggregate
+  # TODO once EV goes in w/ refactoring of getAttestationsForBlock, pull out the getSlot version and use
+  # it. This is incorrect.
+  for attestation in getAttestationsForBlock(pool, state):
+    # getAttestationsForBlock(...) already aggregates
     if attestation.data == attestation_data:
-      # https://github.com/ethereum/eth2.0-specs/blob/v0.10.1/specs/phase0/validator.md#aggregateandproof
+      # https://github.com/ethereum/eth2.0-specs/blob/v0.11.1/specs/phase0/validator.md#aggregateandproof
       return some(AggregateAndProof(
         aggregator_index: index,
         aggregate: attestation,
         selection_proof: slot_signature))
 
-  # TODO in catch-up mode, we could get here, so probably shouldn't assert
-  doAssert false
   none(AggregateAndProof)
diff --git a/beacon_chain/beacon_node.nim b/beacon_chain/beacon_node.nim
index bcc3668f8..a3787214b 100644
--- a/beacon_chain/beacon_node.nim
+++ b/beacon_chain/beacon_node.nim
@@ -15,7 +15,8 @@ import
   conf, time, state_transition, beacon_chain_db, validator_pool, extras,
   attestation_pool, block_pool, eth2_network, eth2_discovery,
   beacon_node_types, mainchain_monitor, version, ssz, ssz/dynamic_navigator,
-  sync_protocol, request_manager, validator_keygen, interop, statusbar
+  sync_protocol, request_manager, validator_keygen, interop, statusbar,
+  attestation_aggregation
 
 const
   genesisFile = "genesis.ssz"
@@ -602,6 +603,45 @@ proc verifyFinalization(node: BeaconNode, slot: Slot) =
       node.blockPool.finalizedHead.blck.slot.compute_epoch_at_slot()
     doAssert finalizedEpoch + 2 == epoch
 
+proc broadcastAggregatedAttestations(
+    node: BeaconNode, state: auto, head: var auto, slot: Slot,
+    trailing_distance: uint64) =
+  # The index is via a
+  # locally attested validator. Unlike in handleAttestations(...) there's a
+  # single one at most per slot (because that's how aggregation attestation
+  # works), so the machinery that has to handle looping across, basically a
+  # set of locally attached validators is in principle not necessary, but a
+  # way to organize this. Then the private key for that validator should be
+  # the corresponding one -- whatver they are, they match.
+
+  let
+    bs = BlockSlot(blck: head, slot: slot)
+    committees_per_slot = get_committee_count_at_slot(state, slot)
+  var cache = get_empty_per_epoch_cache()
+  for committee_index in 0'u64..<committees_per_slot:
+    let
+      committee = get_beacon_committee(state, slot, committee_index, cache)
+
+    for index_in_committee, validatorIdx in committee:
+      let validator = node.getAttachedValidator(state, validatorIdx)
+      if validator != nil:
+        # This is slightly strange/inverted control flow, since really it's
+        # going to happen once per slot, but this is the best way to get at
+        # the validator index and private key pair. TODO verify it only has
+        # one isSome() with test.
+        let option_aggregateandproof =
+          aggregate_attestations(node.attestationPool, state,
+            committee_index,
+            # TODO https://github.com/status-im/nim-beacon-chain/issues/545
+            # this assumes in-process private keys
+            validator.privKey,
+            trailing_distance)
+
+        # Don't broadcast when, e.g., this node isn't an aggregator
+        if option_aggregateandproof.isSome:
+          node.network.broadcast(
+            topicAggregateAndProof, option_aggregateandproof.get)
+
 proc onSlotStart(node: BeaconNode, lastSlot, scheduledSlot: Slot) {.gcsafe, async.} =
   ## Called at the beginning of a slot - usually every slot, but sometimes might
   ## skip a few in case we're running late.
@@ -697,7 +737,7 @@ proc onSlotStart(node: BeaconNode, lastSlot, scheduledSlot: Slot) {.gcsafe, asyn
 
   var head = node.updateHead()
 
-  # TODO is the slot of the clock or the head block more interestion? provide
+  # TODO is the slot of the clock or the head block more interesting? provide
   #      rationale in comment
   beacon_head_slot.set slot.int64
 
@@ -762,11 +802,12 @@ proc onSlotStart(node: BeaconNode, lastSlot, scheduledSlot: Slot) {.gcsafe, asyn
     #      with any clock discrepancies once only, at the start of slot timer
     #      processing..
 
-    # https://github.com/ethereum/eth2.0-specs/blob/v0.9.4/specs/validator/0_beacon-chain-validator.md#attesting
-    # A validator should create and broadcast the attestation to the
-    # associated attestation subnet one-third of the way through the slot
-    # during which the validator is assigned―that is, SECONDS_PER_SLOT / 3
-    # seconds after the start of slot.
+    # https://github.com/ethereum/eth2.0-specs/blob/v0.11.1/specs/phase0/validator.md#attesting
+    # A validator should create and broadcast the attestation to the associated
+    # attestation subnet when either (a) the validator has received a valid
+    # block from the expected block proposer for the assigned slot or
+    # (b) one-third of the slot has transpired (`SECONDS_PER_SLOT / 3` seconds
+    # after the start of slot) -- whichever comes first.
     let
       attestationStart = node.beaconClock.fromNow(slot)
       thirdSlot = seconds(int64(SECONDS_PER_SLOT)) div 3
@@ -788,6 +829,25 @@ proc onSlotStart(node: BeaconNode, lastSlot, scheduledSlot: Slot) {.gcsafe, asyn
 
     handleAttestations(node, head, slot)
 
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.11.1/specs/phase0/validator.md#broadcast-aggregate
+  # If the validator is selected to aggregate (is_aggregator), then they
+  # broadcast their best aggregate as a SignedAggregateAndProof to the global
+  # aggregate channel (beacon_aggregate_and_proof) two-thirds of the way
+  # through the slot-that is, SECONDS_PER_SLOT * 2 / 3 seconds after the start
+  # of slot.
+  if slot > 2:
+    const TRAILING_DISTANCE = 1
+    let aggregationSlot = slot - TRAILING_DISTANCE
+    var aggregationHead = getAncestorAt(head, aggregationSlot)
+
+    let bs = BlockSlot(blck: aggregationHead, slot: aggregationSlot)
+    node.blockPool.withState(node.blockPool.tmpState, bs):
+      let twoThirdsSlot =
+        toBeaconTime(slot, seconds(2*int64(SECONDS_PER_SLOT)) div 3)
+      addTimer(saturate(node.beaconClock.fromNow(twoThirdsSlot))) do (p: pointer):
+        broadcastAggregatedAttestations(
+          node, state, aggregationHead, aggregationSlot, TRAILING_DISTANCE)
+
   # TODO ... and beacon clock might jump here also. sigh.
   let
     nextSlotStart = saturate(node.beaconClock.fromNow(nextSlot))
diff --git a/beacon_chain/spec/network.nim b/beacon_chain/spec/network.nim
index 719237d0c..41076a13d 100644
--- a/beacon_chain/spec/network.nim
+++ b/beacon_chain/spec/network.nim
@@ -13,6 +13,7 @@ const
   topicVoluntaryExits* = "/eth2/voluntary_exit/ssz"
   topicProposerSlashings* = "/eth2/proposer_slashing/ssz"
   topicAttesterSlashings* = "/eth2/attester_slashing/ssz"
+  topicAggregateAndProof* = "/eth2/beacon_aggregate_and_proof/ssz"
 
   # https://github.com/ethereum/eth2.0-specs/blob/v0.11.0/specs/phase0/p2p-interface.md#configuration
   ATTESTATION_SUBNET_COUNT* = 64