diff --git a/lib/selectors/chat-selectors.js b/lib/selectors/chat-selectors.js
index 3f40ad93c..a8f9cda85 100644
--- a/lib/selectors/chat-selectors.js
+++ b/lib/selectors/chat-selectors.js
@@ -1,505 +1,593 @@
 // @flow
 
 import invariant from 'invariant';
 import _filter from 'lodash/fp/filter';
 import _flow from 'lodash/fp/flow';
 import _map from 'lodash/fp/map';
 import _orderBy from 'lodash/fp/orderBy';
 import * as React from 'react';
 import { useSelector } from 'react-redux';
 import { createSelector } from 'reselect';
 import { createObjectSelector } from 'reselect-map';
 
 import {
   messageKey,
   robotextForMessageInfo,
   createMessageInfo,
   getMostRecentNonLocalMessageID,
   sortMessageInfoList,
 } from '../shared/message-utils';
 import {
   threadIsPending,
   threadIsTopLevel,
   threadInChatList,
 } from '../shared/thread-utils';
 import {
   type MessageInfo,
   type MessageStore,
   type ComposableMessageInfo,
   type RobotextMessageInfo,
   type LocalMessageInfo,
   messageTypes,
   isComposableMessageType,
 } from '../types/message-types';
 import type { BaseAppState } from '../types/redux-types';
 import {
   type ThreadInfo,
   type RawThreadInfo,
   type SidebarInfo,
   maxReadSidebars,
   maxUnreadSidebars,
   threadTypes,
 } from '../types/thread-types';
 import type { UserInfo, AccountUserInfo } from '../types/user-types';
 import { threeDays } from '../utils/date-utils';
 import memoize2 from '../utils/memoize';
 import {
   threadInfoSelector,
   sidebarInfoSelector,
   threadInfoFromSourceMessageIDSelector,
 } from './thread-selectors';
 
 export type SidebarItem =
   | {
       ...SidebarInfo,
       +type: 'sidebar',
     }
   | {
       +type: 'seeMore',
       +unread: boolean,
     }
   | { +type: 'spacer' };
 
 export type ChatThreadItem = {
   +type: 'chatThreadItem',
   +threadInfo: ThreadInfo,
   +mostRecentMessageInfo: ?MessageInfo,
   +mostRecentNonLocalMessage: ?string,
   +lastUpdatedTime: number,
   +lastUpdatedTimeIncludingSidebars: number,
   +sidebars: $ReadOnlyArray<SidebarItem>,
   +pendingPersonalThreadUserInfo?: UserInfo,
 };
 
 const messageInfoSelector: (
   state: BaseAppState<*>,
 ) => { +[id: string]: ?MessageInfo } = createObjectSelector(
   (state: BaseAppState<*>) => state.messageStore.messages,
   (state: BaseAppState<*>) => state.currentUserInfo && state.currentUserInfo.id,
   (state: BaseAppState<*>) => state.userStore.userInfos,
   threadInfoSelector,
   createMessageInfo,
 );
 
 function isEmptyMediaMessage(messageInfo: MessageInfo): boolean {
   return (
     (messageInfo.type === messageTypes.MULTIMEDIA ||
       messageInfo.type === messageTypes.IMAGES) &&
     messageInfo.media.length === 0
   );
 }
 
 function getMostRecentMessageInfo(
   threadInfo: ThreadInfo,
   messageStore: MessageStore,
   messages: { +[id: string]: ?MessageInfo },
 ): ?MessageInfo {
   const thread = messageStore.threads[threadInfo.id];
   if (!thread) {
     return null;
   }
   for (const messageID of thread.messageIDs) {
     const messageInfo = messages[messageID];
     if (!messageInfo || isEmptyMediaMessage(messageInfo)) {
       continue;
     }
     return messageInfo;
   }
   return null;
 }
 
 function getLastUpdatedTime(
   threadInfo: ThreadInfo,
   mostRecentMessageInfo: ?MessageInfo,
 ): number {
   return mostRecentMessageInfo
     ? mostRecentMessageInfo.time
     : threadInfo.creationTime;
 }
 
 function createChatThreadItem(
   threadInfo: ThreadInfo,
   messageStore: MessageStore,
   messages: { +[id: string]: ?MessageInfo },
   sidebarInfos: ?$ReadOnlyArray<SidebarInfo>,
 ): ChatThreadItem {
   const mostRecentMessageInfo = getMostRecentMessageInfo(
     threadInfo,
     messageStore,
     messages,
   );
   const mostRecentNonLocalMessage = getMostRecentNonLocalMessageID(
     threadInfo.id,
     messageStore,
   );
   const lastUpdatedTime = getLastUpdatedTime(threadInfo, mostRecentMessageInfo);
 
   const sidebars = sidebarInfos ?? [];
   const allSidebarItems = sidebars.map(sidebarInfo => ({
     type: 'sidebar',
     ...sidebarInfo,
   }));
   const lastUpdatedTimeIncludingSidebars =
     allSidebarItems.length > 0
       ? Math.max(lastUpdatedTime, allSidebarItems[0].lastUpdatedTime)
       : lastUpdatedTime;
 
   const numUnreadSidebars = allSidebarItems.filter(
     sidebar => sidebar.threadInfo.currentUser.unread,
   ).length;
   let numReadSidebarsToShow = maxReadSidebars - numUnreadSidebars;
   const threeDaysAgo = Date.now() - threeDays;
 
   const sidebarItems = [];
   for (const sidebar of allSidebarItems) {
     if (sidebarItems.length >= maxUnreadSidebars) {
       break;
     } else if (sidebar.threadInfo.currentUser.unread) {
       sidebarItems.push(sidebar);
     } else if (
       sidebar.lastUpdatedTime > threeDaysAgo &&
       numReadSidebarsToShow > 0
     ) {
       sidebarItems.push(sidebar);
       numReadSidebarsToShow--;
     }
   }
 
   const numReadButRecentSidebars = allSidebarItems.filter(
     sidebar =>
       !sidebar.threadInfo.currentUser.unread &&
       sidebar.lastUpdatedTime > threeDaysAgo,
   ).length;
   if (
     sidebarItems.length < numUnreadSidebars + numReadButRecentSidebars ||
     (sidebarItems.length < allSidebarItems.length && sidebarItems.length > 0)
   ) {
     sidebarItems.push({
       type: 'seeMore',
       unread: numUnreadSidebars > maxUnreadSidebars,
     });
   }
   if (sidebarItems.length !== 0) {
     sidebarItems.push({
       type: 'spacer',
     });
   }
 
   return {
     type: 'chatThreadItem',
     threadInfo,
     mostRecentMessageInfo,
     mostRecentNonLocalMessage,
     lastUpdatedTime,
     lastUpdatedTimeIncludingSidebars,
     sidebars: sidebarItems,
   };
 }
 
 const chatListData: (
   state: BaseAppState<*>,
 ) => $ReadOnlyArray<ChatThreadItem> = createSelector(
   threadInfoSelector,
   (state: BaseAppState<*>) => state.messageStore,
   messageInfoSelector,
   sidebarInfoSelector,
   (
     threadInfos: { +[id: string]: ThreadInfo },
     messageStore: MessageStore,
     messageInfos: { +[id: string]: ?MessageInfo },
     sidebarInfos: { +[id: string]: $ReadOnlyArray<SidebarInfo> },
   ): $ReadOnlyArray<ChatThreadItem> =>
     getChatThreadItems(
       threadInfos,
       messageStore,
       messageInfos,
       sidebarInfos,
       threadIsTopLevel,
     ),
 );
 
 function useFlattenedChatListData(): $ReadOnlyArray<ChatThreadItem> {
   return useFilteredChatListData(threadInChatList);
 }
 
 function useFilteredChatListData(
   filterFunction: (threadInfo: ?(ThreadInfo | RawThreadInfo)) => boolean,
 ): $ReadOnlyArray<ChatThreadItem> {
   const threadInfos = useSelector(threadInfoSelector);
   const messageInfos = useSelector(messageInfoSelector);
   const sidebarInfos = useSelector(sidebarInfoSelector);
   const messageStore = useSelector(state => state.messageStore);
 
   return React.useMemo(
     () =>
       getChatThreadItems(
         threadInfos,
         messageStore,
         messageInfos,
         sidebarInfos,
         filterFunction,
       ),
     [messageInfos, messageStore, sidebarInfos, filterFunction, threadInfos],
   );
 }
 
 function getChatThreadItems(
   threadInfos: { +[id: string]: ThreadInfo },
   messageStore: MessageStore,
   messageInfos: { +[id: string]: ?MessageInfo },
   sidebarInfos: { +[id: string]: $ReadOnlyArray<SidebarInfo> },
   filterFunction: (threadInfo: ?(ThreadInfo | RawThreadInfo)) => boolean,
 ): $ReadOnlyArray<ChatThreadItem> {
   return _flow(
     _filter(filterFunction),
     _map((threadInfo: ThreadInfo): ChatThreadItem =>
       createChatThreadItem(
         threadInfo,
         messageStore,
         messageInfos,
         sidebarInfos[threadInfo.id],
       ),
     ),
     _orderBy('lastUpdatedTimeIncludingSidebars')('desc'),
   )(threadInfos);
 }
 
 export type RobotextChatMessageInfoItem = {
   +itemType: 'message',
   +messageInfo: RobotextMessageInfo,
   +startsConversation: boolean,
   +startsCluster: boolean,
   endsCluster: boolean,
   +robotext: string,
   +threadCreatedFromMessage: ?ThreadInfo,
+  +reactions: $ReadOnlyMap<string, MessageReactionInfo>,
 };
 export type ChatMessageInfoItem =
   | RobotextChatMessageInfoItem
   | {
       +itemType: 'message',
       +messageInfo: ComposableMessageInfo,
       +localMessageInfo: ?LocalMessageInfo,
       +startsConversation: boolean,
       +startsCluster: boolean,
       endsCluster: boolean,
       +threadCreatedFromMessage: ?ThreadInfo,
+      +reactions: $ReadOnlyMap<string, MessageReactionInfo>,
     };
 export type ChatMessageItem = { itemType: 'loader' } | ChatMessageInfoItem;
+export type MessageReactionInfo = {
+  +viewerReacted: boolean,
+  +users: $ReadOnlySet<string>,
+};
+
 const msInFiveMinutes = 5 * 60 * 1000;
 function createChatMessageItems(
   threadID: string,
   messageStore: MessageStore,
   messageInfos: { +[id: string]: ?MessageInfo },
   threadInfos: { +[id: string]: ThreadInfo },
   threadInfoFromSourceMessageID: { +[id: string]: ThreadInfo },
   additionalMessages: $ReadOnlyArray<MessageInfo>,
+  viewerID: string,
 ): ChatMessageItem[] {
   const thread = messageStore.threads[threadID];
 
   const threadMessageInfos = (thread?.messageIDs ?? [])
     .map((messageID: string) => messageInfos[messageID])
     .filter(Boolean);
   const messages =
     additionalMessages.length > 0
       ? sortMessageInfoList([...threadMessageInfos, ...additionalMessages])
       : threadMessageInfos;
 
+  const targetMessageReactionsMap = new Map<string, Map<string, Set<string>>>();
+
+  // We need to iterate backwards to put the order of messages in chronological
+  // order, starting with the oldest. This avoids the scenario where the most
+  // recent message with the remove_reaction action may try to remove a user
+  // that hasn't been added to the usersSet, causing it to be skipped.
+  for (let i = messages.length - 1; i >= 0; i--) {
+    const messageInfo = messages[i];
+    if (messageInfo.type !== messageTypes.REACTION) {
+      continue;
+    }
+
+    if (!targetMessageReactionsMap.has(messageInfo.targetMessageID)) {
+      const reactsMap = new Map<string, Set<string>>();
+      targetMessageReactionsMap.set(messageInfo.targetMessageID, reactsMap);
+    }
+
+    const messageReactsMap = targetMessageReactionsMap.get(
+      messageInfo.targetMessageID,
+    );
+    invariant(messageReactsMap, 'messageReactsInfo should be set');
+
+    if (!messageReactsMap.has(messageInfo.reaction)) {
+      const usersSet = new Set<string>();
+      messageReactsMap.set(messageInfo.reaction, usersSet);
+    }
+
+    const messageReactionUsersSet = messageReactsMap.get(messageInfo.reaction);
+    invariant(messageReactionUsersSet, 'messageReactionUsersSet should be set');
+
+    if (messageInfo.action === 'add_reaction') {
+      messageReactionUsersSet.add(messageInfo.creator.id);
+    } else {
+      messageReactionUsersSet.delete(messageInfo.creator.id);
+    }
+  }
+
   const chatMessageItems = [];
   let lastMessageInfo = null;
   for (let i = messages.length - 1; i >= 0; i--) {
     const messageInfo = messages[i];
 
     if (messageInfo.type === messageTypes.REACTION) {
       continue;
     }
 
     const originalMessageInfo =
       messageInfo.type === messageTypes.SIDEBAR_SOURCE
         ? messageInfo.sourceMessage
         : messageInfo;
 
     if (isEmptyMediaMessage(originalMessageInfo)) {
       continue;
     }
 
     let startsConversation = true;
     let startsCluster = true;
     if (
       lastMessageInfo &&
       lastMessageInfo.time + msInFiveMinutes > originalMessageInfo.time
     ) {
       startsConversation = false;
       if (
         isComposableMessageType(lastMessageInfo.type) &&
         isComposableMessageType(originalMessageInfo.type) &&
         lastMessageInfo.creator.id === originalMessageInfo.creator.id
       ) {
         startsCluster = false;
       }
     }
     if (startsCluster && chatMessageItems.length > 0) {
       const lastMessageItem = chatMessageItems[chatMessageItems.length - 1];
       invariant(lastMessageItem.itemType === 'message', 'should be message');
       lastMessageItem.endsCluster = true;
     }
     const threadCreatedFromMessage =
       messageInfo.id && threadInfos[threadID]?.type !== threadTypes.SIDEBAR
         ? threadInfoFromSourceMessageID[messageInfo.id]
         : undefined;
+
+    const renderedReactions: $ReadOnlyMap<
+      string,
+      MessageReactionInfo,
+    > = (() => {
+      const result = new Map<string, MessageReactionInfo>();
+
+      let messageReactsMap;
+      if (originalMessageInfo.id) {
+        messageReactsMap = targetMessageReactionsMap.get(
+          originalMessageInfo.id,
+        );
+      }
+
+      if (!messageReactsMap) {
+        return result;
+      }
+
+      for (const reaction of messageReactsMap.keys()) {
+        const reactionUsersSet = messageReactsMap.get(reaction);
+        invariant(reactionUsersSet, 'reactionUsersSet should be set');
+
+        if (reactionUsersSet.size === 0) {
+          continue;
+        }
+
+        const messageReactionInfo = {
+          users: reactionUsersSet,
+          viewerReacted: reactionUsersSet.has(viewerID),
+        };
+
+        result.set(reaction, messageReactionInfo);
+      }
+
+      return result;
+    })();
+
     if (isComposableMessageType(originalMessageInfo.type)) {
       // We use these invariants instead of just checking the messageInfo.type
       // directly in the conditional above so that isComposableMessageType can
       // be the source of truth
       invariant(
         originalMessageInfo.type === messageTypes.TEXT ||
           originalMessageInfo.type === messageTypes.IMAGES ||
           originalMessageInfo.type === messageTypes.MULTIMEDIA,
         "Flow doesn't understand isComposableMessageType above",
       );
       const localMessageInfo =
         messageStore.local[messageKey(originalMessageInfo)];
       chatMessageItems.push({
         itemType: 'message',
         messageInfo: originalMessageInfo,
         localMessageInfo,
         startsConversation,
         startsCluster,
         endsCluster: false,
         threadCreatedFromMessage,
+        reactions: renderedReactions,
       });
     } else {
       invariant(
         originalMessageInfo.type !== messageTypes.TEXT &&
           originalMessageInfo.type !== messageTypes.IMAGES &&
           originalMessageInfo.type !== messageTypes.MULTIMEDIA,
         "Flow doesn't understand isComposableMessageType above",
       );
       const robotext = robotextForMessageInfo(
         originalMessageInfo,
         threadInfos[threadID],
       );
       chatMessageItems.push({
         itemType: 'message',
         messageInfo: originalMessageInfo,
         startsConversation,
         startsCluster,
         endsCluster: false,
         threadCreatedFromMessage,
         robotext,
+        reactions: renderedReactions,
       });
     }
     lastMessageInfo = originalMessageInfo;
   }
   if (chatMessageItems.length > 0) {
     const lastMessageItem = chatMessageItems[chatMessageItems.length - 1];
     invariant(lastMessageItem.itemType === 'message', 'should be message');
     lastMessageItem.endsCluster = true;
   }
   chatMessageItems.reverse();
   const hideSpinner = thread ? thread.startReached : threadIsPending(threadID);
   if (hideSpinner) {
     return chatMessageItems;
   }
   return [...chatMessageItems, ({ itemType: 'loader' }: ChatMessageItem)];
 }
 
 const baseMessageListData = (
   threadID: ?string,
   additionalMessages: $ReadOnlyArray<MessageInfo>,
 ) =>
   createSelector(
     (state: BaseAppState<*>) => state.messageStore,
     messageInfoSelector,
     threadInfoSelector,
     threadInfoFromSourceMessageIDSelector,
+    (state: BaseAppState<*>) =>
+      state.currentUserInfo && state.currentUserInfo.id,
     (
       messageStore: MessageStore,
       messageInfos: { +[id: string]: ?MessageInfo },
       threadInfos: { +[id: string]: ThreadInfo },
       threadInfoFromSourceMessageID: { +[id: string]: ThreadInfo },
+      viewerID: ?string,
     ): ?(ChatMessageItem[]) => {
-      if (!threadID) {
+      if (!threadID || !viewerID) {
         return null;
       }
       return createChatMessageItems(
         threadID,
         messageStore,
         messageInfos,
         threadInfos,
         threadInfoFromSourceMessageID,
         additionalMessages,
+        viewerID,
       );
     },
   );
 type MessageListData = ?(ChatMessageItem[]);
 const messageListData: (
   threadID: ?string,
   additionalMessages: $ReadOnlyArray<MessageInfo>,
 ) => (state: BaseAppState<*>) => MessageListData = memoize2(
   baseMessageListData,
 );
 
 type UseMessageListDataArgs = {
   +searching: boolean,
   +userInfoInputArray: $ReadOnlyArray<AccountUserInfo>,
   +threadInfo: ?ThreadInfo,
 };
 
 function useMessageListData({
   searching,
   userInfoInputArray,
   threadInfo,
 }: UseMessageListDataArgs): MessageListData {
   const pendingSidebarSourceMessageInfo = useSelector(state => {
     const sourceMessageID = threadInfo?.sourceMessageID;
     if (
       !threadInfo ||
       threadInfo.type !== threadTypes.SIDEBAR ||
       !sourceMessageID
     ) {
       return null;
     }
     const thread = state.messageStore.threads[threadInfo.id];
     const messageInfos = messageInfoSelector(state);
     const shouldSourceBeAdded =
       !thread ||
       (thread.startReached &&
         thread.messageIDs.every(
           id => messageInfos[id]?.type !== messageTypes.SIDEBAR_SOURCE,
         ));
     return shouldSourceBeAdded ? messageInfos[sourceMessageID] : null;
   });
 
   invariant(
     !pendingSidebarSourceMessageInfo ||
       pendingSidebarSourceMessageInfo.type !== messageTypes.SIDEBAR_SOURCE,
     'sidebars can not be created from sidebar_source message',
   );
 
   const additionalMessages = React.useMemo(
     () =>
       pendingSidebarSourceMessageInfo ? [pendingSidebarSourceMessageInfo] : [],
     [pendingSidebarSourceMessageInfo],
   );
   const boundMessageListData = useSelector(
     messageListData(threadInfo?.id, additionalMessages),
   );
 
   return React.useMemo(() => {
     if (searching && userInfoInputArray.length === 0) {
       return [];
     }
     return boundMessageListData;
   }, [searching, userInfoInputArray.length, boundMessageListData]);
 }
 
 export {
   messageInfoSelector,
   createChatThreadItem,
   chatListData,
   createChatMessageItems,
   messageListData,
   useFlattenedChatListData,
   useFilteredChatListData,
   useMessageListData,
 };