diff --git a/lib/matrix_server/room_server.ex b/lib/matrix_server/room_server.ex
index 0338727..304c18c 100644
--- a/lib/matrix_server/room_server.ex
+++ b/lib/matrix_server/room_server.ex
@@ -114,6 +114,14 @@ defmodule MatrixServer.RoomServer do
     GenServer.call(pid, {:join, account})
   end
 
+  @doc """
+  Leave a room.
+  """
+  @spec leave(pid(), Account.t()) :: :ok | {:error, atom()}
+  def leave(pid, account) do
+    GenServer.call(pid, {:leave, account})
+  end
+
   ### Implementation
 
   @impl true
@@ -218,6 +226,29 @@ defmodule MatrixServer.RoomServer do
     end
   end
 
+  def handle_call({:leave, account}, _from, %{room: room, state_set: state_set} = state) do
+    case Repo.transaction(leave_insert_event(room, state_set, account)) do
+      {:ok, state_set} -> {:reply, :ok, %{state | state_set: state_set}}
+      {:error, reason} -> {:reply, {:error, reason}, state}
+    end
+  end
+
+  @spec leave_insert_event(Room.t(), t(), Account.t()) :: (() -> {:ok, t()} | {:error, atom()})
+  defp leave_insert_event(room, state_set, account) do
+    leave_event = Event.leave(room, account)
+
+    fn ->
+      case finalize_and_insert_event(leave_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 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
diff --git a/lib/matrix_server/schema/account.ex b/lib/matrix_server/schema/account.ex
index 9fc4ce6..b91f2bf 100644
--- a/lib/matrix_server/schema/account.ex
+++ b/lib/matrix_server/schema/account.ex
@@ -5,7 +5,7 @@ defmodule MatrixServer.Account do
 
   alias MatrixServer.{Repo, Account, Device, Room, JoinedRoom}
   alias MatrixServerWeb.Client.Request.{Register, Login}
-  alias Ecto.Multi
+  alias Ecto.{Multi, Changeset}
 
   @type t :: %__MODULE__{
           password_hash: String.t()
@@ -25,6 +25,10 @@ defmodule MatrixServer.Account do
     timestamps(updated_at: false)
   end
 
+  @doc """
+  Reports whether the given user localpart is available on this server.
+  """
+  @spec available?(String.t()) :: :ok | {:error, :user_in_use | :invalid_username}
   def available?(localpart) when is_binary(localpart) do
     if Regex.match?(MatrixServer.localpart_regex(), localpart) and
          String.length(localpart) <= localpart_length() do
@@ -42,6 +46,10 @@ defmodule MatrixServer.Account do
     end
   end
 
+  @doc """
+  Return an multi to register a new user.
+  """
+  @spec register(Register.t()) :: Multi.t()
   def register(%Register{} = input) do
     account_params = %{
       localpart: input.username || MatrixServer.random_string(10, ?a..?z),
@@ -62,6 +70,10 @@ defmodule MatrixServer.Account do
     |> Multi.run(:device_with_access_token, &Device.insert_new_access_token/2)
   end
 
+  @doc """
+  Return a function to log a user in.
+  """
+  @spec login(Login.t()) :: (Ecto.Repo.t() -> {:error, any()} | {:ok, Device.t()})
   def login(%Login{} = input) do
     localpart = try_get_localpart(input.identifier.user)
 
@@ -86,6 +98,10 @@ defmodule MatrixServer.Account do
     end
   end
 
+  @doc """
+  Get a device and its associated account using the device's access token.
+  """
+  @spec by_access_token(String.t()) :: {Account.t(), Device.t()} | nil
   def by_access_token(access_token) do
     Device
     |> where([d], d.access_token == ^access_token)
@@ -94,6 +110,7 @@ defmodule MatrixServer.Account do
     |> Repo.one()
   end
 
+  @spec changeset(map(), map()) :: Changeset.t()
   def changeset(account, params \\ %{}) do
     # TODO: fix password_hash in params
     account
@@ -105,11 +122,13 @@ defmodule MatrixServer.Account do
     |> unique_constraint(:localpart, name: :accounts_pkey)
   end
 
+  @spec localpart_length :: integer()
   defp localpart_length do
     # Subtract the "@" and ":" in the MXID.
     @max_mxid_length - 2 - String.length(MatrixServer.server_name())
   end
 
+  @spec try_get_localpart(String.t()) :: String.t()
   defp try_get_localpart("@" <> rest = user_id) do
     case String.split(rest, ":", parts: 2) do
       [localpart, _] -> localpart
@@ -118,4 +137,12 @@ defmodule MatrixServer.Account do
   end
 
   defp try_get_localpart(localpart), do: localpart
+
+  @doc """
+  Get the matrix user ID of an account.
+  """
+  @spec get_mxid(Account.t()) :: String.t()
+  def get_mxid(%Account{localpart: localpart}) do
+    "@" <> localpart <> ":" <> MatrixServer.server_name()
+  end
 end
diff --git a/lib/matrix_server/schema/device.ex b/lib/matrix_server/schema/device.ex
index 99d919a..1099f17 100644
--- a/lib/matrix_server/schema/device.ex
+++ b/lib/matrix_server/schema/device.ex
@@ -6,6 +6,13 @@ defmodule MatrixServer.Device do
   alias MatrixServer.{Account, Device, Repo}
   alias MatrixServerWeb.Client.Request.Login
 
+  @type t :: %__MODULE__{
+          device_id: String.t(),
+          access_token: String.t(),
+          display_name: String.t(),
+          localpart: String.t()
+        }
+
   @primary_key false
   schema "devices" do
     field :device_id, :string, primary_key: true
diff --git a/lib/matrix_server/schema/event.ex b/lib/matrix_server/schema/event.ex
index 62a6ffb..2e3ee7a 100644
--- a/lib/matrix_server/schema/event.ex
+++ b/lib/matrix_server/schema/event.ex
@@ -201,6 +201,18 @@ defmodule MatrixServer.Event do
     }
   end
 
+  @spec leave(Room.t(), Account.t()) :: t()
+  def leave(room, sender) do
+    %Event{
+      new(room, sender)
+      | type: "m.room.member",
+        state_key: Account.get_mxid(sender),
+        content: %{
+          "membership" => "leave"
+        }
+    }
+  end
+
   @spec is_control_event(t()) :: boolean()
   def is_control_event(%Event{type: "m.room.power_levels", state_key: ""}), do: true
   def is_control_event(%Event{type: "m.room.join_rules", state_key: ""}), do: true
diff --git a/lib/matrix_server/state_resolution/authorization.ex b/lib/matrix_server/state_resolution/authorization.ex
index c643d1d..3fc970b 100644
--- a/lib/matrix_server/state_resolution/authorization.ex
+++ b/lib/matrix_server/state_resolution/authorization.ex
@@ -76,19 +76,6 @@ defmodule MatrixServer.StateResolution.Authorization do
     end
   end
 
-  def authorized?(
-        %Event{
-          type: "m.room.member",
-          sender: sender,
-          content: %{"membership" => "leave"},
-          state_key: sender
-        },
-        state_set
-      ) do
-    # Check rule: 5.4.1
-    get_membership(to_string(sender), state_set) in ["invite", "join"]
-  end
-
   def authorized?(
         %Event{
           type: "m.room.member",
@@ -99,24 +86,30 @@ defmodule MatrixServer.StateResolution.Authorization do
         state_set
       ) do
     sender_membership = get_membership(to_string(sender), state_set)
-    target_membership = get_membership(state_key, state_set)
-    power_levels = get_power_levels(state_set)
-    sender_pl = get_user_power_level(to_string(sender), power_levels)
-    target_pl = get_user_power_level(state_key, power_levels)
 
-    # Check rules: 5.4.2, 5.4.3, 5.4.4
-    cond do
-      sender_membership != "join" ->
-        false
+    if to_string(sender) == state_key do
+      # Check rule: 5.4.1
+      sender_membership in ["invite", "join"]
+    else
+      target_membership = get_membership(state_key, state_set)
+      power_levels = get_power_levels(state_set)
+      sender_pl = get_user_power_level(to_string(sender), power_levels)
+      target_pl = get_user_power_level(state_key, power_levels)
 
-      target_membership == "ban" and not has_power_level?(to_string(sender), power_levels, :ban) ->
-        false
+      # Check rules: 5.4.2, 5.4.3, 5.4.4
+      cond do
+        sender_membership != "join" ->
+          false
 
-      has_power_level?(to_string(sender), power_levels, :kick) and target_pl < sender_pl ->
-        true
+        target_membership == "ban" and not has_power_level?(to_string(sender), power_levels, :ban) ->
+          false
 
-      true ->
-        false
+        has_power_level?(to_string(sender), power_levels, :kick) and target_pl < sender_pl ->
+          true
+
+        true ->
+          false
+      end
     end
   end
 
@@ -312,6 +305,7 @@ defmodule MatrixServer.StateResolution.Authorization do
       |> Repo.all()
       |> Enum.reduce(%{}, &update_state_set/2)
 
+    IO.inspect(event)
     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 bb94754..0870d7f 100644
--- a/lib/matrix_server_web/client/controllers/room_controller.ex
+++ b/lib/matrix_server_web/client/controllers/room_controller.ex
@@ -112,4 +112,29 @@ defmodule MatrixServerWeb.Client.RoomController do
   end
 
   def join(conn, _), do: put_error(conn, :missing_param)
+
+  @doc """
+  This API stops a user participating in a particular room.
+
+  Action for POST /_matrix/client/r0/rooms/{roomId}/leave.
+  """
+  def leave(%Conn{assigns: %{account: account}} = conn, %{"room_id" => room_id}) do
+    case RoomServer.get_room_server(room_id) do
+      {:ok, pid} ->
+        case RoomServer.leave(pid, account) do
+          :ok ->
+            conn
+            |> send_resp(200, [])
+            |> halt()
+
+          {:error, _} ->
+            put_error(conn, :unknown)
+        end
+
+      {:error, :not_found} ->
+        put_error(conn, :not_found, "The given room was not found.")
+    end
+  end
+
+  def leave(conn, _), do: put_error(conn, :missing_param)
 end
diff --git a/lib/matrix_server_web/client/request/login.ex b/lib/matrix_server_web/client/request/login.ex
index 5e5d5c1..e824a8d 100644
--- a/lib/matrix_server_web/client/request/login.ex
+++ b/lib/matrix_server_web/client/request/login.ex
@@ -3,6 +3,13 @@ defmodule MatrixServerWeb.Client.Request.Login do
 
   import Ecto.Changeset
 
+  @type t :: %__MODULE__{
+          type: String.t(),
+          password: String.t(),
+          device_id: String.t(),
+          initial_device_display_name: String.t()
+        }
+
   @primary_key false
   embedded_schema do
     field :type, :string
diff --git a/lib/matrix_server_web/client/request/register.ex b/lib/matrix_server_web/client/request/register.ex
index 2969a76..545f138 100644
--- a/lib/matrix_server_web/client/request/register.ex
+++ b/lib/matrix_server_web/client/request/register.ex
@@ -5,6 +5,14 @@ defmodule MatrixServerWeb.Client.Request.Register do
 
   alias Ecto.Changeset
 
+  @type t :: %__MODULE__{
+          device_id: String.t(),
+          initial_device_display_name: String.t(),
+          password: String.t(),
+          username: String.t(),
+          inhibit_login: boolean()
+        }
+
   @primary_key false
   embedded_schema do
     field :device_id, :string
diff --git a/lib/matrix_server_web/router.ex b/lib/matrix_server_web/router.ex
index bb60f87..6f1075d 100644
--- a/lib/matrix_server_web/router.ex
+++ b/lib/matrix_server_web/router.ex
@@ -60,6 +60,7 @@ defmodule MatrixServerWeb.Router do
       scope "/rooms/:room_id" do
         post "/invite", RoomController, :invite
         post "/join", RoomController, :join
+        post "/leave", RoomController, :leave
       end
     end
   end