diff --git a/lib/architex/schema/event/formatters.ex b/lib/architex/schema/event/formatters.ex
index d195dc5..84b7ea1 100644
--- a/lib/architex/schema/event/formatters.ex
+++ b/lib/architex/schema/event/formatters.ex
@@ -4,8 +4,8 @@ defmodule Architex.Event.Formatters do
   """
   alias Architex.Event
 
-  @spec for_client(Event.t()) :: map()
-  def for_client(%Event{
+  @spec messages_response(Event.t()) :: map()
+  def messages_response(%Event{
         content: content,
         type: type,
         id: event_id,
@@ -30,6 +30,27 @@ defmodule Architex.Event.Formatters do
     data
   end
 
+  def sync_response(%Event{
+        content: content,
+        type: type,
+        id: event_id,
+        sender: sender,
+        origin_server_ts: origin_server_ts,
+        unsigned: unsigned
+      }) do
+    data = %{
+      content: content,
+      type: type,
+      event_id: event_id,
+      sender: to_string(sender),
+      origin_server_ts: origin_server_ts
+    }
+
+    data = if unsigned, do: Map.put(data, :unsigned, unsigned), else: data
+
+    data
+  end
+
   @spec as_pdu(Event.t()) :: map()
   def as_pdu(%Event{
         auth_events: auth_events,
diff --git a/lib/architex/schema/room.ex b/lib/architex/schema/room.ex
index c27a88f..d07413f 100644
--- a/lib/architex/schema/room.ex
+++ b/lib/architex/schema/room.ex
@@ -70,8 +70,6 @@ defmodule Architex.Room do
 
   @spec get_messages(Room.t(), Messages.t()) :: {[Event.t()], integer() | nil, integer() | nil}
   def get_messages(room, %Messages{from: from, to: to, dir: dir, limit: limit}) do
-    limit = limit || 10
-
     events =
       room
       |> Ecto.assoc(:events)
@@ -90,6 +88,8 @@ defmodule Architex.Room do
 
   # When 'from' is empty, we return events from the start or end
   # of the room's history.
+  # TODO: Might actually not be needed, could be that Quaternion is passing
+  # an empty string because the /sync call fails.
   @spec events_from(Ecto.Query.t(), String.t(), String.t()) :: Ecto.Query.t()
   defp events_from(query, "", _), do: query
 
diff --git a/lib/architex_web/client/controllers/room_controller.ex b/lib/architex_web/client/controllers/room_controller.ex
index 3a9b244..207c531 100644
--- a/lib/architex_web/client/controllers/room_controller.ex
+++ b/lib/architex_web/client/controllers/room_controller.ex
@@ -249,7 +249,7 @@ defmodule ArchitexWeb.Client.RoomController do
       case Repo.one(room_query) do
         %Room{} = room ->
           {events, start, end_} = Room.get_messages(room, request)
-          events = Enum.map(events, &Event.Formatters.for_client/1)
+          events = Enum.map(events, &Event.Formatters.messages_response/1)
           data = %{chunk: events}
           data = if start, do: Map.put(data, :start, Integer.to_string(start)), else: data
           data = if end_, do: Map.put(data, :end, Integer.to_string(end_)), else: data
diff --git a/lib/architex_web/client/controllers/sync_controller.ex b/lib/architex_web/client/controllers/sync_controller.ex
new file mode 100644
index 0000000..ce6d2c9
--- /dev/null
+++ b/lib/architex_web/client/controllers/sync_controller.ex
@@ -0,0 +1,73 @@
+defmodule ArchitexWeb.Client.SyncController do
+  use ArchitexWeb, :controller
+
+  import ArchitexWeb.Error
+  import Ecto.Query
+
+  alias Architex.{Repo, Event, Account, Room, JoinedRoom}
+  alias Plug.Conn
+
+  @doc """
+  Synchronise the client's state with the latest state on the server.
+
+  Parameters: %{"filter" => "{\"account_data\":{},\"presence\":{},\"room\":{\"account_data\":{},\"ephemeral\":{},\"state\":{\"lazy_load_members\":true},\"timeline\":{\"limit\":100}}}", "path" => ["_matrix", "client", "r0", "sync"], "timeout" => "30000"}
+  Action for GET /_matrix/client/r0/sync.
+  """
+  # When no "since" is specified, return the most recent messages.
+  def sync(%Conn{assigns: %{account: %Account{id: account_id}}} = conn, params)
+      when not is_map_key(params, "since") do
+    # joined_rooms =
+    #   account
+    #   |> Ecto.assoc(:joined_rooms)
+    #   |> Repo.all()
+    #   |> Enum.into(%{}, fn %Room{id: room_id} = room ->
+    #     {room_id, room}
+    #   end)
+
+    events_per_room =
+      Event
+      |> join(:inner, [e], jr in JoinedRoom,
+        on: jr.room_id == e.room_id and jr.account_id == ^account_id
+      )
+      |> join(:inner, [e, jr], r in Room, on: r.id == jr.room_id)
+      |> order_by(asc: :origin_server_ts, asc: :nid)
+      |> Repo.all()
+      |> Enum.group_by(& &1.room_id)
+
+    join =
+      Enum.into(events_per_room, %{}, fn {room_id, [%Event{nid: first_nid} | _] = events} ->
+        joined_room = %{
+          timeline: %{
+            events: Enum.map(events, &Event.Formatters.sync_response/1),
+            limited: false,
+            prev_batch: Integer.to_string(first_nid)
+          }
+        }
+
+        {room_id, joined_room}
+      end)
+
+    next_batch = Enum.map(events_per_room, fn {_, events} ->
+      %Event{nid: last_nid} = List.last(events)
+      last_nid
+    end)
+    |> Enum.max(fn -> 0 end)
+
+    data = %{
+      next_batch: Integer.to_string(next_batch),
+      rooms: %{
+        join: join,
+        invite: %{},
+        leave: %{}
+      }
+    }
+
+    conn
+    |> put_status(200)
+    |> json(data)
+  end
+
+  # TODO: Long-poll for new incoming events.
+  # Should think about how to implement this in a nice way.
+  def sync(conn, _params), do: put_error(conn, :unknown, "Not implemented yet.")
+end
diff --git a/lib/architex_web/client/request/messages.ex b/lib/architex_web/client/request/messages.ex
index e6b80d9..346e14d 100644
--- a/lib/architex_web/client/request/messages.ex
+++ b/lib/architex_web/client/request/messages.ex
@@ -2,19 +2,19 @@ defmodule ArchitexWeb.Client.Request.Messages do
   use ArchitexWeb.Request
 
   @type t :: %__MODULE__{
-    from: String.t(),
-    to: String.t() | nil,
-    dir: String.t(),
-    limit: integer() | nil,
-    filter: String.t() | nil
-  }
+          from: String.t(),
+          to: String.t() | nil,
+          dir: String.t(),
+          limit: integer() | nil,
+          filter: String.t() | nil
+        }
 
   @primary_key false
   embedded_schema do
     field :from, :string
     field :to, :string
     field :dir, :string
-    field :limit, :integer
+    field :limit, :integer, default: 10
     field :filter, :string
   end
 
diff --git a/lib/architex_web/client/request/sync.ex b/lib/architex_web/client/request/sync.ex
new file mode 100644
index 0000000..f73bd4f
--- /dev/null
+++ b/lib/architex_web/client/request/sync.ex
@@ -0,0 +1,14 @@
+defmodule ArchitexWeb.Client.Request.Sync do
+  use ArchitexWeb.Request
+
+  @type t :: %__MODULE__{}
+
+  @primary_key false
+  embedded_schema do
+    field :filter, :string
+    field :since, :string
+    field :full_state, :boolean, default: false
+    field :set_presence, :string, default: "online"
+    field :timeout, :integer, default: 0
+  end
+end
diff --git a/lib/architex_web/router.ex b/lib/architex_web/router.ex
index 015d0f2..eb99026 100644
--- a/lib/architex_web/router.ex
+++ b/lib/architex_web/router.ex
@@ -54,6 +54,7 @@ defmodule ArchitexWeb.Router do
       post "/createRoom", RoomController, :create
       get "/joined_rooms", RoomController, :joined_rooms
       get "/capabilities", InfoController, :capabilities
+      get "/sync", SyncController, :sync
 
       scope "/directory" do
         put "/room/:alias", AliasesController, :create