diff --git a/src/Scylla/Room.elm b/src/Scylla/Room.elm
new file mode 100644
index 0000000..532ba82
--- /dev/null
+++ b/src/Scylla/Room.elm
@@ -0,0 +1,110 @@
+module Scylla.Room exposing (..)
+import Scylla.Route exposing (RoomId)
+import Scylla.Sync exposing (SyncResponse)
+import Scylla.Sync.Events exposing (MessageEvent, StateEvent, toStateEvent, toMessageEvent)
+import Scylla.Sync.AccountData exposing (AccountData)
+import Scylla.Sync.Rooms exposing (JoinedRoom, UnreadNotificationCounts, Ephemeral)
+import Json.Decode as Decode exposing (Value)
+import Dict exposing (Dict)
+
+type alias RoomState = Dict (String, String) Value
+
+type alias RoomData =
+    { roomState : RoomState
+    , messages : List (MessageEvent)
+    , accountData : AccountData
+    , ephemeral : Ephemeral
+    , unreadNotifications : UnreadNotificationCounts
+    , text : String
+    }
+
+type alias OpenRooms = Dict RoomId RoomData
+
+emptyOpenRooms : OpenRooms
+emptyOpenRooms = Dict.empty
+
+emptyRoomData : RoomData
+emptyRoomData =
+    { roomState = Dict.empty
+    , messages = []
+    , accountData = { events = Just [] }
+    , ephemeral = { events = Just [] }
+    , unreadNotifications =
+        { highlightCount = Just 0
+        , notificationCount = Just 0
+        }
+    , text = ""
+    }
+
+changeRoomStateEvent : StateEvent -> RoomState -> RoomState
+changeRoomStateEvent se = Dict.insert (se.type_, se.stateKey) se.content
+
+changeRoomStateEvents : List StateEvent -> RoomState -> RoomState
+changeRoomStateEvents es rs = List.foldr (changeRoomStateEvent) rs es
+
+changeRoomState : JoinedRoom -> RoomState -> RoomState
+changeRoomState jr rs =
+    let
+        stateDiff = jr.state
+            |> Maybe.andThen .events
+            |> Maybe.withDefault []
+        timelineDiff = jr.timeline
+            |> Maybe.andThen .events
+            |> Maybe.map (List.filterMap toStateEvent)
+            |> Maybe.withDefault []
+    in
+        rs
+            |> changeRoomStateEvents stateDiff
+            |> changeRoomStateEvents timelineDiff
+
+changeAccountData : JoinedRoom -> AccountData -> AccountData
+changeAccountData jr ad =
+    case jr.accountData of
+        Nothing -> ad
+        Just newAd ->
+            case (newAd.events, ad.events) of
+                (Just es, Nothing) -> newAd
+                (Just newEs, Just es) -> { events = Just (newEs ++ es) }
+                _ -> ad
+
+changeTimeline : JoinedRoom -> List (MessageEvent) -> List (MessageEvent)
+changeTimeline jr tl =
+    let
+        newMessages = jr.timeline
+            |> Maybe.andThen .events
+            |> Maybe.map (List.filterMap toMessageEvent)
+            |> Maybe.withDefault []
+    in
+        newMessages ++ tl
+
+changeEphemeral : JoinedRoom -> Ephemeral -> Ephemeral
+changeEphemeral jr e = Maybe.withDefault e jr.ephemeral
+
+changeNotifications : JoinedRoom -> UnreadNotificationCounts -> UnreadNotificationCounts
+changeNotifications jr un = Maybe.withDefault un jr.unreadNotifications
+
+changeRoomData : JoinedRoom -> RoomData -> RoomData
+changeRoomData jr rd =
+    { rd | accountData = changeAccountData jr rd.accountData
+    , roomState = changeRoomState jr rd.roomState
+    , messages = changeTimeline jr rd.messages
+    , ephemeral = changeEphemeral jr rd.ephemeral
+    , unreadNotifications = changeNotifications jr rd.unreadNotifications
+    }
+
+updateRoomData : JoinedRoom -> Maybe RoomData -> Maybe RoomData
+updateRoomData jr mrd = Maybe.withDefault emptyRoomData mrd
+    |> changeRoomData jr
+    |> Just
+
+applyJoinedRoom : RoomId -> JoinedRoom -> OpenRooms -> OpenRooms
+applyJoinedRoom rid jr = Dict.update rid (updateRoomData jr)
+
+applySync : SyncResponse -> OpenRooms -> OpenRooms
+applySync sr or =
+    let
+        joinedRooms = sr.rooms
+            |> Maybe.andThen .join
+            |> Maybe.withDefault Dict.empty
+    in
+        Dict.foldl applyJoinedRoom or joinedRooms