diff --git a/lib/matrix_server/room_server.ex b/lib/matrix_server/room_server.ex
index 29a7d27..7e8be49 100644
--- a/lib/matrix_server/room_server.ex
+++ b/lib/matrix_server/room_server.ex
@@ -107,6 +107,14 @@ defmodule MatrixServer.RoomServer do
     GenServer.call(pid, {:invite, account, user_id})
   end
 
+  @doc """
+  Join a room.
+  """
+  @spec join(pid(), Account.t()) :: {:ok, String.t()} | {:error, atom()}
+  def join(pid, account) do
+    GenServer.call(pid, {:join, account})
+  end
+
   ### Implementation
 
   @impl true
@@ -200,6 +208,30 @@ defmodule MatrixServer.RoomServer do
     end
   end
 
+  def handle_call({:join, account}, _from, %{room: %Room{id: room_id} = room, state_set: state_set} = state) do
+    case Repo.transaction(join_insert_event(room, state_set, account)) do
+      {:ok, state_set} -> {:reply, {:ok, room_id}, %{state | state_set: state_set}}
+      {:error, reason} -> {:reply, {:error, reason}, state}
+    end
+  end
+
+  # Get a function that inserts a join event into the room for the given account.
+  @spec join_insert_event(Room.t(), t(), Account.t()) :: (-> {:ok, t()} | {:error, atom()})
+  defp join_insert_event(room, state_set, account) do
+    join_event = Event.join(room, account)
+
+    fn ->
+      case finalize_and_insert_event(join_event, state_set, room) do
+        {:ok, state_set, room} ->
+          _ = update_room_state_set(room, state_set)
+          state_set
+
+        {:error, reason} ->
+          Repo.rollback(reason)
+      end
+    end
+  end
+
   # Get a function that inserts an invite event into a room.
   @spec invite_insert_event(Room.t(), t(), Account.t(), String.t()) ::
           (() -> {:ok, t()} | {:error, atom()})
diff --git a/lib/matrix_server/state_resolution/authorization.ex b/lib/matrix_server/state_resolution/authorization.ex
index 68ec447..c643d1d 100644
--- a/lib/matrix_server/state_resolution/authorization.ex
+++ b/lib/matrix_server/state_resolution/authorization.ex
@@ -312,9 +312,6 @@ defmodule MatrixServer.StateResolution.Authorization do
       |> Repo.all()
       |> Enum.reduce(%{}, &update_state_set/2)
 
-    IO.inspect(event)
-    IO.inspect(state_set)
-
     authorized?(event, state_set)
   end
 end
diff --git a/lib/matrix_server_web/client/controllers/room_controller.ex b/lib/matrix_server_web/client/controllers/room_controller.ex
index 0b537ac..bb94754 100644
--- a/lib/matrix_server_web/client/controllers/room_controller.ex
+++ b/lib/matrix_server_web/client/controllers/room_controller.ex
@@ -86,4 +86,30 @@ defmodule MatrixServerWeb.Client.RoomController do
   end
 
   def invite(conn, _), do: put_error(conn, :missing_param)
+
+  @doc """
+  This API starts a user participating in a particular room, if that user is allowed to participate in that room.
+
+  Action for POST /_matrix/client/r0/rooms/{roomId}/join.
+  TODO: third_party_signed
+  """
+  def join(%Conn{assigns: %{account: account}} = conn, %{"room_id" => room_id}) do
+    case RoomServer.get_room_server(room_id) do
+      {:ok, pid} ->
+        case RoomServer.join(pid, account) do
+          {:ok, room_id} ->
+            conn
+            |> put_status(200)
+            |> json(%{room_id: room_id})
+
+          {:error, _} ->
+            put_error(conn, :unknown)
+        end
+
+      {:error, :not_found} ->
+        put_error(conn, :not_found, "The given room was not found.")
+    end
+  end
+
+  def join(conn, _), do: put_error(conn, :missing_param)
 end
diff --git a/lib/matrix_server_web/router.ex b/lib/matrix_server_web/router.ex
index f82fc7d..bb60f87 100644
--- a/lib/matrix_server_web/router.ex
+++ b/lib/matrix_server_web/router.ex
@@ -59,6 +59,7 @@ defmodule MatrixServerWeb.Router do
 
       scope "/rooms/:room_id" do
         post "/invite", RoomController, :invite
+        post "/join", RoomController, :join
       end
     end
   end