diff --git a/substrate/tendermint/machine/src/ext.rs b/substrate/tendermint/machine/src/ext.rs
index b9ffe5cc..270a3b12 100644
--- a/substrate/tendermint/machine/src/ext.rs
+++ b/substrate/tendermint/machine/src/ext.rs
@@ -121,7 +121,7 @@ impl<S: SignatureScheme> SignatureScheme for Arc<S> {
 /// a valid commit.
 #[derive(Clone, PartialEq, Debug, Encode, Decode)]
 pub struct Commit<S: SignatureScheme> {
-  /// End time of the round, used as the start time of next round.
+  /// End time of the round which created this commit, used as the start time of the next block.
   pub end_time: u64,
   /// Validators participating in the signature.
   pub validators: Vec<S::ValidatorId>,
diff --git a/substrate/tendermint/machine/src/lib.rs b/substrate/tendermint/machine/src/lib.rs
index 6e45f7e2..6b674ae5 100644
--- a/substrate/tendermint/machine/src/lib.rs
+++ b/substrate/tendermint/machine/src/lib.rs
@@ -119,6 +119,7 @@ pub struct TendermintMachine<N: Network> {
   msg_recv: mpsc::UnboundedReceiver<
     SignedMessage<N::ValidatorId, N::Block, <N::SignatureScheme as SignatureScheme>::Signature>,
   >,
+  step_recv: mpsc::UnboundedReceiver<(Commit<N::SignatureScheme>, N::Block)>,
 
   log: MessageLog<N>,
   slashes: HashSet<N::ValidatorId>,
@@ -142,6 +143,9 @@ pub type MessageSender<N> = mpsc::UnboundedSender<
 
 /// A Tendermint machine and its channel to receive messages from the gossip layer over.
 pub struct TendermintHandle<N: Network> {
+  /// Channel to trigger the machine to move to the next height.
+  /// Takes in the the previous block's commit, along with the new proposal.
+  pub step: mpsc::UnboundedSender<(Commit<N::SignatureScheme>, N::Block)>,
   /// Channel to send messages received from the P2P layer.
   pub messages: MessageSender<N>,
   /// Tendermint machine to be run on an asynchronous task.
@@ -251,6 +255,26 @@ impl<N: Network + 'static> TendermintMachine<N> {
     self.round(Round(0));
   }
 
+  async fn reset_by_commit(&mut self, commit: Commit<N::SignatureScheme>, proposal: N::Block) {
+    // Determine the Round number this commit ended on
+    let mut round = Round(0);
+    // Use < to prevent an infinite loop
+    while self.canonical_end_time(round) < commit.end_time {
+      round.0 += 1;
+    }
+    debug_assert_eq!(
+      self.canonical_end_time(round),
+      commit.end_time,
+      "resetting by commit for a different block"
+    );
+
+    // Populate the various pieces of round info
+    if self.round.0 < round.0 {
+      self.round(round);
+    }
+    self.reset(round, proposal).await;
+  }
+
   async fn slash(&mut self, validator: N::ValidatorId) {
     if !self.slashes.contains(&validator) {
       self.slashes.insert(validator);
@@ -268,7 +292,9 @@ impl<N: Network + 'static> TendermintMachine<N> {
     proposal: N::Block,
   ) -> TendermintHandle<N> {
     let (msg_send, msg_recv) = mpsc::unbounded();
+    let (step_send, step_recv) = mpsc::unbounded();
     TendermintHandle {
+      step: step_send,
       messages: msg_send,
       machine: {
         let last_end = UNIX_EPOCH + Duration::from_secs(last.1);
@@ -317,6 +343,7 @@ impl<N: Network + 'static> TendermintMachine<N> {
 
           queue: VecDeque::new(),
           msg_recv,
+          step_recv,
 
           log: MessageLog::new(weights),
           slashes: HashSet::new(),
@@ -364,6 +391,16 @@ impl<N: Network + 'static> TendermintMachine<N> {
         if self.queue.is_empty() { Fuse::terminated() } else { future::ready(()).fuse() };
 
       if let Some((broadcast, msg)) = futures::select_biased! {
+        // Handle a new height occuring externally (an external sync loop)
+        msg = self.step_recv.next() => {
+          if let Some((commit, proposal)) = msg {
+            self.reset_by_commit(commit, proposal).await;
+            None
+          } else {
+            break;
+          }
+        },
+
         // Handle our messages
         _ = queue_future => {
           Some((true, self.queue.pop_front().unwrap()))
diff --git a/substrate/tendermint/machine/tests/ext.rs b/substrate/tendermint/machine/tests/ext.rs
index 3e3e52ce..62552ff7 100644
--- a/substrate/tendermint/machine/tests/ext.rs
+++ b/substrate/tendermint/machine/tests/ext.rs
@@ -152,7 +152,7 @@ impl TestNetwork {
       let mut write = arc.write().await;
       for i in 0 .. validators {
         let i = u16::try_from(i).unwrap();
-        let TendermintHandle { messages, machine } = TendermintMachine::new(
+        let TendermintHandle { messages, machine, .. } = TendermintMachine::new(
           TestNetwork(i, arc.clone()),
           (BlockNumber(1), (SystemTime::now().duration_since(UNIX_EPOCH)).unwrap().as_secs()),
           TestBlock { id: 1u32.to_le_bytes(), valid: Ok(()) },