diff --git a/coordinator/tributary/tendermint/src/lib.rs b/coordinator/tributary/tendermint/src/lib.rs
index cd676b13a..9927473ca 100644
--- a/coordinator/tributary/tendermint/src/lib.rs
+++ b/coordinator/tributary/tendermint/src/lib.rs
@@ -3,7 +3,7 @@ use core::fmt::Debug;
 use std::{
   sync::Arc,
   time::{SystemTime, Instant, Duration},
-  collections::VecDeque,
+  collections::{VecDeque, HashMap},
 };
 
 use parity_scale_codec::{Encode, Decode};
@@ -245,6 +245,10 @@ pub struct TendermintMachine<N: Network> {
   synced_block_result_send: mpsc::UnboundedSender<bool>,
 
   block: BlockData<N>,
+  // TODO: Move this into the Block struct
+  round_proposals: HashMap<RoundNumber, (Option<RoundNumber>, N::Block)>,
+  // TODO: Move this into the Round struct
+  upons: Upons,
 }
 
 pub struct SyncedBlock<N: Network> {
@@ -346,6 +350,9 @@ impl<N: Network + 'static> TendermintMachine<N> {
       proposal,
     );
 
+    // Reset the round proposals
+    self.round_proposals = HashMap::new();
+
     // Start the first round
     self.round(RoundNumber(0), Some(round_end));
   }
@@ -383,6 +390,410 @@ impl<N: Network + 'static> TendermintMachine<N> {
     }
   }
 
+  fn proposal_for_round(&self, round: RoundNumber) -> Option<(Option<RoundNumber>, &N::Block)> {
+    self.round_proposals.get(&round).map(|(round, block)| (*round, block))
+  }
+
+  // L22-27
+  fn upon_proposal_without_valid_round(&mut self) {
+    if self.block.round().step != Step::Propose {
+      return;
+    }
+
+    // If we have the proposal message...
+    let Some((None, block)) = self.proposal_for_round(self.block.round().number) else {
+      return;
+    };
+
+    // There either needs to not be a locked value or it must be equivalent
+    #[allow(clippy::map_unwrap_or)]
+    if self
+      .block
+      .locked
+      .as_ref()
+      .map(|(_round, locked_block)| block.id() == *locked_block)
+      .unwrap_or(true)
+    {
+      self.broadcast(Data::Prevote(Some(block.id())));
+    } else {
+      self.broadcast(Data::Prevote(None));
+    }
+  }
+
+  // L28-33
+  fn upon_proposal_with_valid_round(&mut self) {
+    if self.block.round().step != Step::Propose {
+      return;
+    }
+
+    // If we have the proposal message...
+    let Some((Some(proposal_valid_round), block)) =
+      self.proposal_for_round(self.block.round().number)
+    else {
+      return;
+    };
+
+    // Check we have the necessary prevotes
+    if !self.block.log.has_consensus(proposal_valid_round, &Data::Prevote(Some(block.id()))) {
+      return;
+    }
+
+    // We don't check valid round < current round as the `message` function does
+
+    // If locked is None, lockedRoundp is -1 and less than valid round
+    #[allow(clippy::map_unwrap_or)]
+    let locked_clause_1 = self
+      .block
+      .locked
+      .as_ref()
+      .map(|(locked_round, _block)| locked_round.0 <= proposal_valid_round.0)
+      .unwrap_or(true);
+    // The second clause is if the locked values are equivalent. If no value is locked, they aren't
+    #[allow(clippy::map_unwrap_or)]
+    let locked_clause_2 = self
+      .block
+      .locked
+      .as_ref()
+      .map(|(_round, locked_block)| block.id() == *locked_block)
+      .unwrap_or(false);
+
+    if locked_clause_1 || locked_clause_2 {
+      self.broadcast(Data::Prevote(Some(block.id())));
+    } else {
+      self.broadcast(Data::Prevote(None));
+    }
+  }
+
+  // L34-35
+  fn upon_prevotes(&mut self) {
+    if self.upons.upon_prevotes || (self.block.round().step != Step::Prevote) {
+      return;
+    }
+
+    if self.block.log.has_participation(self.block.round().number, Step::Prevote) {
+      self.block.round_mut().set_timeout(Step::Prevote);
+      self.upons.upon_prevotes = true;
+    }
+  }
+
+  // L36-43
+  async fn upon_successful_current_round_prevotes(&mut self) {
+    // Returning if `self.step == Step::Propose` is equivalent to guarding `step >= prevote`
+    if self.upons.upon_successful_current_round_prevotes ||
+      (self.block.round().step == Step::Propose)
+    {
+      return;
+    }
+
+    // If we have the proposal message...
+    let Some((_, block)) = self.proposal_for_round(self.block.round().number) else {
+      return;
+    };
+
+    // Check we have the necessary prevotes
+    if !self.block.log.has_consensus(self.block.round().number, &Data::Prevote(Some(block.id()))) {
+      return;
+    }
+
+    let block = block.clone();
+    self.upons.upon_successful_current_round_prevotes = true;
+
+    if self.block.round().step == Step::Prevote {
+      self.block.locked = Some((self.block.round().number, block.id()));
+      let signature = self
+        .signer
+        .sign(&commit_msg(
+          self.block.end_time[&self.block.round().number].canonical(),
+          block.id().as_ref(),
+        ))
+        .await;
+      self.broadcast(Data::Precommit(Some((block.id(), signature))));
+    }
+    self.block.valid = Some((self.block.round().number, block));
+  }
+
+  // L44-46
+  fn upon_negative_current_round_prevotes(&mut self) {
+    if self.upons.upon_negative_current_round_prevotes || (self.block.round().step != Step::Prevote)
+    {
+      return;
+    }
+
+    if self.block.log.has_consensus(self.block.round().number, &Data::Prevote(None)) {
+      self.broadcast(Data::Precommit(None));
+    }
+
+    self.upons.upon_negative_current_round_prevotes = true;
+  }
+
+  // L47-48
+  fn upon_precommits(&mut self) {
+    if self.upons.upon_precommits {
+      return;
+    }
+
+    if self.block.log.has_participation(self.block.round().number, Step::Precommit) {
+      self.block.round_mut().set_timeout(Step::Precommit);
+      self.upons.upon_precommits = true;
+    }
+  }
+
+  // L22-48
+  async fn all_current_round_upons(&mut self) {
+    self.upon_proposal_without_valid_round();
+    self.upon_proposal_with_valid_round();
+    self.upon_prevotes();
+    self.upon_successful_current_round_prevotes().await;
+    self.upon_negative_current_round_prevotes();
+    self.upon_precommits();
+  }
+
+  // L49-54
+  async fn upon_successful_precommits(&mut self, round: RoundNumber) -> bool {
+    // If we have the proposal message...
+    let Some((_, block)) = self.proposal_for_round(round) else { return false };
+
+    // Check we have the necessary precommits
+    // The precommit we check we have consensus upon uses a junk signature since message equality
+    // disregards the signature
+    if !self
+      .block
+      .log
+      .has_consensus(round, &Data::Precommit(Some((block.id(), self.signer.sign(&[]).await))))
+    {
+      return false;
+    }
+
+    // Get all participants in this commit
+    let mut validators = vec![];
+    let mut sigs = vec![];
+    // Get all precommits for this round
+    for (validator, msgs) in &self.block.log.log[&round] {
+      if let Some(signed) = msgs.get(&Step::Precommit) {
+        if let Data::Precommit(Some((id, sig))) = &signed.msg.data {
+          // If this precommit was for this block, include it
+          if *id == block.id() {
+            validators.push(*validator);
+            sigs.push(sig.clone());
+          }
+        }
+      }
+    }
+
+    // Form the commit itself
+    let commit_msg = commit_msg(self.block.end_time[&round].canonical(), block.id().as_ref());
+    let commit = Commit {
+      end_time: self.block.end_time[&round].canonical(),
+      validators: validators.clone(),
+      signature: self.network.signature_scheme().aggregate(&validators, &commit_msg, &sigs),
+    };
+    debug_assert!(self.network.verify_commit(block.id(), &commit));
+
+    // Add the block and reset the machine
+    log::info!(
+      target: "tendermint",
+      "TendermintMachine produced block {}",
+      hex::encode(block.id().as_ref()),
+    );
+    let id = block.id();
+    let proposal = self.network.add_block(block.clone(), commit).await;
+    log::trace!(
+      target: "tendermint",
+      "added block {} (produced by machine)",
+      hex::encode(id.as_ref()),
+    );
+    self.reset(round, proposal).await;
+
+    true
+  }
+
+  // L49-54
+  async fn all_any_round_upons(&mut self, round: RoundNumber) -> bool {
+    self.upon_successful_precommits(round).await
+  }
+
+  // Returns Ok(true) if this was a Precommit which had either no signature or its signature
+  // validated
+  // Returns Ok(false) if it wasn't a Precommit or the signature wasn't validated yet
+  // Returns Err if the signature was invalid
+  async fn verify_precommit_signature(
+    &mut self,
+    signed: &SignedMessageFor<N>,
+  ) -> Result<bool, TendermintError> {
+    let msg = &signed.msg;
+    if let Data::Precommit(precommit) = &msg.data {
+      let Some((id, sig)) = precommit else { return Ok(true) };
+      // Also verify the end_time of the commit
+      // Only perform this verification if we already have the end_time
+      // Else, there's a DoS where we receive a precommit for some round infinitely in the future
+      // which forces us to calculate every end time
+      if let Some(end_time) = self.block.end_time.get(&msg.round) {
+        if !self.validators.verify(msg.sender, &commit_msg(end_time.canonical(), id.as_ref()), sig)
+        {
+          log::warn!(target: "tendermint", "validator produced an invalid commit signature");
+          self
+            .slash(
+              msg.sender,
+              SlashEvent::WithEvidence(Evidence::InvalidPrecommit(signed.encode())),
+            )
+            .await;
+          Err(TendermintError::Malicious)?;
+        }
+        return Ok(true);
+      }
+    }
+    Ok(false)
+  }
+
+  async fn message(&mut self, signed: &SignedMessageFor<N>) -> Result<(), TendermintError> {
+    let msg = &signed.msg;
+    if msg.block != self.block.number {
+      Err(TendermintError::Temporal)?;
+    }
+
+    // If this is a precommit, verify its signature
+    self.verify_precommit_signature(signed).await?;
+
+    // Only let the proposer propose
+    if matches!(msg.data, Data::Proposal(..)) &&
+      (msg.sender != self.weights.proposer(msg.block, msg.round))
+    {
+      log::warn!(target: "tendermint", "validator who wasn't the proposer proposed");
+      // TODO: This should have evidence
+      self
+        .slash(msg.sender, SlashEvent::Id(SlashReason::InvalidProposer, msg.block.0, msg.round.0))
+        .await;
+      Err(TendermintError::Malicious)?;
+    };
+
+    // If this is a proposal, verify the block
+    // If the block is invalid, drop the message, letting the timeout cover it
+    // This prevents needing to check if valid inside every `upon` block
+    if let Data::Proposal(_, block) = &msg.data {
+      match self.network.validate(block).await {
+        Ok(()) => {}
+        Err(BlockError::Temporal) => return Err(TendermintError::Temporal),
+        Err(BlockError::Fatal) => {
+          log::warn!(target: "tendermint", "validator proposed a fatally invalid block");
+          self
+            .slash(
+              msg.sender,
+              SlashEvent::Id(SlashReason::InvalidBlock, self.block.number.0, msg.round.0),
+            )
+            .await;
+          Err(TendermintError::Malicious)?;
+        }
+      };
+    }
+
+    // If this is a proposal, verify the valid round isn't fundamentally invalid
+    if let Data::Proposal(Some(valid_round), _) = msg.data {
+      if valid_round.0 >= msg.round.0 {
+        log::warn!(
+          target: "tendermint",
+          "proposed proposed with a syntactically invalid valid round",
+        );
+        self
+          .slash(msg.sender, SlashEvent::WithEvidence(Evidence::InvalidValidRound(msg.encode())))
+          .await;
+        Err(TendermintError::Malicious)?;
+      }
+    }
+
+    // Add it to the log, returning if it was already handled
+    match self.block.log.log(signed.clone()) {
+      Ok(true) => {}
+      Ok(false) => Err(TendermintError::AlreadyHandled)?,
+      Err(evidence) => {
+        self.slash(msg.sender, SlashEvent::WithEvidence(evidence)).await;
+        Err(TendermintError::Malicious)?;
+      }
+    }
+    log::debug!(
+      target: "tendermint",
+      "received new tendermint message (block: {}, round: {}, step: {:?})",
+      msg.block.0,
+      msg.round.0,
+      msg.data.step(),
+    );
+
+    // If this is a proposal, insert it
+    if let Data::Proposal(vr, block) = &msg.data {
+      self.round_proposals.insert(msg.round, (*vr, block.clone()));
+    }
+
+    // L55-56
+    // Jump ahead if we should
+    if (msg.round.0 > self.block.round().number.0) &&
+      (self.block.log.round_participation(msg.round) >= self.weights.fault_threshold())
+    {
+      log::debug!(
+        target: "tendermint",
+        "jumping from round {} to round {}",
+        self.block.round().number.0,
+        msg.round.0,
+      );
+
+      // Jump to the new round.
+      let old_round = self.block.round().number;
+      self.round(msg.round, None);
+
+      // If any jumped over/to round already has precommit messages, verify their signatures
+      for jumped in (old_round.0 + 1) ..= msg.round.0 {
+        let jumped = RoundNumber(jumped);
+        let round_msgs = self.block.log.log.get(&jumped).cloned().unwrap_or_default();
+        for (validator, msgs) in &round_msgs {
+          if let Some(existing) = msgs.get(&Step::Precommit) {
+            if let Ok(res) = self.verify_precommit_signature(existing).await {
+              // Ensure this actually verified the signature instead of believing it shouldn't yet
+              assert!(res);
+            } else {
+              // Remove the message so it isn't counted towards forming a commit/included in one
+              // This won't remove the fact they precommitted for this block hash in the MessageLog
+              // TODO: Don't even log these in the first place until we jump, preventing needing
+              // to do this in the first place
+              self
+                .block
+                .log
+                .log
+                .get_mut(&jumped)
+                .unwrap()
+                .get_mut(validator)
+                .unwrap()
+                .remove(&Step::Precommit)
+                .unwrap();
+            }
+          }
+        }
+      }
+    }
+
+    // Now that we've jumped, and:
+    // 1) If this is a message for an old round, verified the precommit signatures
+    // 2) If this is a message for what was the current round, verified the precommit signatures
+    // 3) If this is a message for what was a future round, verified the precommit signatures if it
+    //    has 34+% participation
+    // Run all `upons` run for any round, which may produce a Commit if it has 67+% participation
+    // (returning true if it does, letting us return now)
+    // It's necessary to verify the precommit signatures before Commit production is allowed, hence
+    // this specific flow
+    if self.all_any_round_upons(msg.round).await {
+      return Ok(());
+    }
+
+    // If this is a historic round, or a future round without sufficient participation, return
+    if msg.round.0 != self.block.round().number.0 {
+      return Ok(());
+    }
+    // msg.round is now guaranteed to be equal to self.block.round().number
+    debug_assert_eq!(msg.round, self.block.round().number);
+
+    // Run all `upons` run for the current round
+    self.all_current_round_upons().await;
+
+    Ok(())
+  }
+
   /// Create a new Tendermint machine, from the specified point, with the specified block as the
   /// one to propose next. This will return a channel to send messages from the gossip layer and
   /// the machine itself. The machine should have `run` called from an asynchronous task.
@@ -450,6 +861,15 @@ impl<N: Network + 'static> TendermintMachine<N> {
             validator_id,
             Some(proposal),
           ),
+
+          round_proposals: HashMap::new(),
+
+          upons: Upons {
+            upon_prevotes: false,
+            upon_successful_current_round_prevotes: false,
+            upon_negative_current_round_prevotes: false,
+            upon_precommits: false,
+          },
         };
 
         // The end time of the last block is the start time for this one
diff --git a/coordinator/tributary/tendermint/src/message_log.rs b/coordinator/tributary/tendermint/src/message_log.rs
index 3959852d8..a150617be 100644
--- a/coordinator/tributary/tendermint/src/message_log.rs
+++ b/coordinator/tributary/tendermint/src/message_log.rs
@@ -86,13 +86,4 @@ impl<N: Network> MessageLog<N> {
     let (_, weight) = self.message_instances(round, data);
     weight >= self.weights.threshold()
   }
-
-  pub(crate) fn get(
-    &self,
-    round: RoundNumber,
-    sender: N::ValidatorId,
-    step: Step,
-  ) -> Option<&SignedMessageFor<N>> {
-    self.log.get(&round).and_then(|round| round.get(&sender).and_then(|msgs| msgs.get(&step)))
-  }
 }