diff --git a/lib/matrix_server.ex b/lib/matrix_server.ex
index e77af16..29adb31 100644
--- a/lib/matrix_server.ex
+++ b/lib/matrix_server.ex
@@ -1,4 +1,8 @@
 defmodule MatrixServer do
+  @moduledoc """
+  Utility functions used throughout the project.
+  """
+
   alias MatrixServer.EncodableMap
 
   @random_string_alphabet Enum.into(?a..?z, []) ++ Enum.into(?A..?Z, [])
@@ -6,30 +10,49 @@ defmodule MatrixServer do
   @ipv4_regex ~r/^(?<hostname>[^:]+)(?<port>:\d{1,5})?$/
   @dns_regex ~r/^[[:alnum:]-.]{1,255}$/
 
+  @doc """
+  Get the full MXID for a user's localpart, using the homeserver's
+  server name as the domain.
+  """
   @spec get_mxid(String.t()) :: String.t()
   def get_mxid(localpart) when is_binary(localpart) do
     "@#{localpart}:#{server_name()}"
   end
 
+  @doc """
+  Get the homeserver's server name.
+  """
   @spec server_name() :: String.t()
   def server_name do
     Application.get_env(:matrix_server, :server_name)
   end
 
+  @doc """
+  Get a regex to match a user's localpart.
+  """
   @spec localpart_regex() :: Regex.t()
   def localpart_regex, do: ~r/^([a-z0-9\._=\/])+$/
 
-  @spec random_string(pos_integer()) :: String.t()
-  def random_string(length), do: random_string(length, @random_string_alphabet)
-
+  @doc """
+  Get a random string of `length` length and using `alphabet`'s characters.
+  """
   @spec random_string(pos_integer(), Enum.t()) :: String.t()
-  def random_string(length, alphabet) when length >= 1 do
+  def random_string(length, alphabet \\ @random_string_alphabet) when length >= 1 do
     for _ <- 1..length, into: "", do: <<Enum.random(alphabet)>>
   end
 
+  @doc """
+  Get the homeserver's default room version.
+  """
   @spec default_room_version() :: String.t()
   def default_room_version, do: "7"
 
+  @doc """
+  Get the domain from an ID.
+
+  Return `nil` if no domain was found.
+  No validation on the domain is performed whatsoever.
+  """
   @spec get_domain(String.t()) :: String.t() | nil
   def get_domain(id) do
     case String.split(id, ":", parts: 2) do
@@ -38,6 +61,12 @@ defmodule MatrixServer do
     end
   end
 
+  @doc """
+  Get the localpart from a MXID.
+
+  Return `nil` if no localpart was found.
+  No validation on the localpart is performed whatsoever.
+  """
   @spec get_localpart(String.t()) :: String.t() | nil
   def get_localpart(id) do
     with [part, _] <- String.split(id, ":", parts: 2),
@@ -48,9 +77,12 @@ defmodule MatrixServer do
     end
   end
 
-  # https://elixirforum.com/t/22709/9
+  @doc """
+  Check whether the given list contains duplicates.
+  """
   @spec has_duplicates?(list()) :: boolean()
   def has_duplicates?(list) do
+    # https://elixirforum.com/t/22709/9
     list
     |> Enum.reduce_while(%MapSet{}, fn x, acc ->
       if MapSet.member?(acc, x), do: {:halt, false}, else: {:cont, MapSet.put(acc, x)}
@@ -58,15 +90,25 @@ defmodule MatrixServer do
     |> is_boolean()
   end
 
-  # https://matrix.org/docs/spec/appendices#unpadded-base64
+  @doc """
+  Encode the given string using unpadded base64.
+
+  Unpadded base64 is the same as base64, except the "=" padding is removed.
+  """
   @spec encode_unpadded_base64(String.t()) :: String.t()
   def encode_unpadded_base64(data) do
+    # https://matrix.org/docs/spec/appendices#unpadded-base64
     data
     |> Base.encode64()
     |> String.trim_trailing("=")
   end
 
-  # Decode (possibly unpadded) base64.
+  @doc """
+  Decode the given base64 string.
+
+  The base64 is allowed to be unpadded, as specified in `encode_unpadded_base64`.
+  Return `:error` on decode error.
+  """
   @spec decode_base64(String.t()) :: {:ok, String.t()} | :error
   def decode_base64(data) when is_binary(data) do
     rem = rem(String.length(data), 4)
@@ -74,6 +116,13 @@ defmodule MatrixServer do
     Base.decode64(padded_data)
   end
 
+  @doc """
+  Encode the given map as canonical JSON.
+
+  See [the Matrix docs](https://matrix.org/docs/spec/appendices#canonical-json) for explanation
+  for canonical JSON.
+  Return an error if the map could not be encoded.
+  """
   @spec encode_canonical_json(map()) :: {:ok, String.t()} | {:error, Jason.EncodeError.t()}
   def encode_canonical_json(object) do
     object
@@ -81,9 +130,13 @@ defmodule MatrixServer do
     |> Jason.encode()
   end
 
-  # https://stackoverflow.com/questions/41523762/41671211
+  @doc """
+  Convert a struct to a map, removing any schema or association fields,
+  as well as fields that have `nil` values.
+  """
   @spec to_serializable_map(struct()) :: map()
   def to_serializable_map(struct) do
+    # https://stackoverflow.com/questions/41523762/41671211
     association_fields =
       if Kernel.function_exported?(struct.__struct__, :__schema__, 1) do
         struct.__struct__.__schema__(:associations)
@@ -101,6 +154,9 @@ defmodule MatrixServer do
     |> Enum.into(%{})
   end
 
+  @doc """
+  Serialize and encode the given struct.
+  """
   @spec serialize_and_encode(struct()) :: {:ok, String.t()} | {:error, Jason.EncodeError.t()}
   def serialize_and_encode(struct) do
     struct
@@ -108,6 +164,11 @@ defmodule MatrixServer do
     |> encode_canonical_json()
   end
 
+  @doc """
+  Add a signature to the given map under the `:signatures` key.
+
+  If the map has no `:signatures` key, it is created.
+  """
   @spec add_signature(map(), String.t(), String.t()) :: map()
   def add_signature(object, key_id, sig) when not is_map_key(object, :signatures) do
     Map.put(object, :signatures, %{MatrixServer.server_name() => %{key_id => sig}})
@@ -120,6 +181,9 @@ defmodule MatrixServer do
     %{object | signatures: new_sigs}
   end
 
+  @doc """
+  Validate a changeset's field where the reason for invalidation is not needed.
+  """
   @spec validate_change_simple(Ecto.Changeset.t(), atom(), (term() -> boolean())) ::
           Ecto.Changeset.t()
   def validate_change_simple(changeset, field, func) do
@@ -130,8 +194,11 @@ defmodule MatrixServer do
     Ecto.Changeset.validate_change(changeset, field, augmented_func)
   end
 
-  # Returns a Boolean whether the signature is valid.
-  # Also returns false on ArgumentError.
+  @doc """
+  Return a Boolean whether the given signature is valid.
+
+  Returns `false` if `:enacl` throws an exception.
+  """
   @spec sign_verify(binary(), String.t(), binary()) :: boolean()
   def sign_verify(sig, text, key) do
     try do
@@ -141,6 +208,9 @@ defmodule MatrixServer do
     end
   end
 
+  @doc """
+  Get the earliest of the two given `DateTime`s.
+  """
   @spec min_datetime(DateTime.t(), DateTime.t()) :: DateTime.t()
   def min_datetime(datetime1, datetime2) do
     if DateTime.compare(datetime1, datetime2) == :gt do
@@ -150,6 +220,13 @@ defmodule MatrixServer do
     end
   end
 
+  @doc """
+  Encode the given string using URL-safe base64.
+
+  URL-safe base64 is the same was unpadded base64, except "+" is replaced by "-"
+  and "/" is replaced by "_".
+  See [the Matrix docs](https://spec.matrix.org/unstable/rooms/v4/) for more details.
+  """
   @spec encode_url_safe_base64(String.t()) :: String.t()
   def encode_url_safe_base64(data) do
     data
@@ -158,6 +235,12 @@ defmodule MatrixServer do
     |> String.replace("/", "_")
   end
 
+  @doc """
+  Check whether the given domain (including port) is valid.
+
+  The domain could be a DNS name, IPv4 or IPv6 address.
+  See [the Matrix docs](https://matrix.org/docs/spec/appendices#server-name) for more details.
+  """
   @spec valid_domain?(String.t()) :: boolean()
   def valid_domain?(domain) do
     if String.starts_with?(domain, "[") do
diff --git a/lib/matrix_server/key_server.ex b/lib/matrix_server/key_server.ex
index 39b4e07..fb4ba83 100644
--- a/lib/matrix_server/key_server.ex
+++ b/lib/matrix_server/key_server.ex
@@ -1,4 +1,10 @@
 defmodule MatrixServer.KeyServer do
+  @moduledoc """
+  A GenServer holding the homeserver's keys, and responsible for signing objects.
+
+  Currently, it only supports one key pair that cannot expire.
+  """
+
   use GenServer
 
   # TODO: only support one signing key for now.
@@ -10,11 +16,22 @@ defmodule MatrixServer.KeyServer do
     GenServer.start_link(__MODULE__, opts, name: __MODULE__)
   end
 
+  @doc """
+  Sign the given object using the homeserver's signing keys.
+
+  Return the signature and the key ID used.
+  On error, return `:error`.
+  """
   @spec sign_object(map()) :: {:ok, String.t(), String.t()} | :error
   def sign_object(object) do
     GenServer.call(__MODULE__, {:sign_object, object})
   end
 
+  @doc """
+  Get the homeserver's signing keys.
+
+  Return a list of tuples, each holding the key ID and the key itself.
+  """
   @spec get_own_signing_keys() :: list({String.t(), binary()})
   def get_own_signing_keys() do
     GenServer.call(__MODULE__, :get_own_signing_keys)
diff --git a/lib/matrix_server/room_server.ex b/lib/matrix_server/room_server.ex
index a9a393a..be58038 100644
--- a/lib/matrix_server/room_server.ex
+++ b/lib/matrix_server/room_server.ex
@@ -1,4 +1,11 @@
 defmodule MatrixServer.RoomServer do
+  @moduledoc """
+  A GenServer to hold and manipulate the state of a Matrix room.
+
+  Each RoomServer corresponds to one Matrix room that the homeserver participates in.
+  The RoomServers are supervised by a DynamicSupervisor RoomServer.Supervisor.
+  """
+
   use GenServer
 
   import Ecto.Query
@@ -18,17 +25,16 @@ defmodule MatrixServer.RoomServer do
     GenServer.start_link(__MODULE__, opts, name: name)
   end
 
-  @spec get_room_server(Room.t()) :: {:error, :not_found} | DynamicSupervisor.on_start_child()
-  def get_room_server(%Room{id: room_id}), do: get_room_server(room_id)
+  @doc """
+  Get the PID of the RoomServer for a room.
 
-  # Get room server pid, or spin one up for the room.
-  # If the room does not exist, return an error.
+  If the given room has no running RoomServer yet, it is created.
+  If the given room does not exist, an error is returned.
+  """
   @spec get_room_server(String.t()) :: {:error, :not_found} | DynamicSupervisor.on_start_child()
   def get_room_server(room_id) do
+    # TODO: Might be wise to use a transaction here to prevent race conditions.
     case Repo.one(from r in Room, where: r.id == ^room_id) do
-      nil ->
-        {:error, :not_found}
-
       %Room{state: serialized_state_set} ->
         case Registry.lookup(@registry, room_id) do
           [{pid, _}] ->
@@ -43,9 +49,19 @@ defmodule MatrixServer.RoomServer do
 
             DynamicSupervisor.start_child(@supervisor, {__MODULE__, opts})
         end
+
+      nil ->
+        {:error, :not_found}
     end
   end
 
+  @doc """
+  Create a new Matrix room.
+
+  The new room is created with the given `account` as creator.
+  Events are inserted into the new room depending on the input `input` and according
+  to the [Matrix documentation](https://matrix.org/docs/spec/client_server/r0.6.1#post-matrix-client-r0-createroom).
+  """
   @spec create_room(
           pid(),
           MatrixServer.Account.t(),
@@ -55,16 +71,30 @@ defmodule MatrixServer.RoomServer do
     GenServer.call(pid, {:create_room, account, input})
   end
 
-  @spec server_in_room(pid(), String.t()) :: boolean()
-  def server_in_room(pid, domain) do
-    GenServer.call(pid, {:server_in_room, domain})
+  @doc """
+  Check whether the given server participates in a room.
+
+  Check whether any participant of the room has a server name matching
+  the given `domain`.
+  """
+  @spec server_in_room?(pid(), String.t()) :: boolean()
+  def server_in_room?(pid, domain) do
+    GenServer.call(pid, {:server_in_room?, domain})
   end
 
+  @doc """
+  Get the state of a room, before the given event was inserted.
+
+  Return a list of all state events and the auth chain.
+  """
   @spec get_state_at_event(pid(), Event.t()) :: {[Event.t()], [Event.t()]}
   def get_state_at_event(pid, event) do
     GenServer.call(pid, {:get_state_at_event, event})
   end
 
+  @doc """
+  Same as `get_state_at_event/2`, except returns the lists as event IDs.
+  """
   @spec get_state_ids_at_event(pid(), Event.t()) :: {[String.t()], [String.t()]}
   def get_state_ids_at_event(pid, event) do
     GenServer.call(pid, {:get_state_ids_at_event, event})
@@ -101,7 +131,7 @@ defmodule MatrixServer.RoomServer do
     end
   end
 
-  def handle_call({:server_in_room, domain}, _from, %{state_set: state_set} = state) do
+  def handle_call({:server_in_room?, domain}, _from, %{state_set: state_set} = state) do
     result =
       Enum.any?(state_set, fn
         {{"m.room.member", user_id}, %Event{content: %{"membership" => "join"}}} ->
diff --git a/lib/matrix_server_web/federation/controllers/event_controller.ex b/lib/matrix_server_web/federation/controllers/event_controller.ex
index 5a7d05e..3f1ed6f 100644
--- a/lib/matrix_server_web/federation/controllers/event_controller.ex
+++ b/lib/matrix_server_web/federation/controllers/event_controller.ex
@@ -18,7 +18,7 @@ defmodule MatrixServerWeb.Federation.EventController do
       %Event{room: room} = event ->
         case RoomServer.get_room_server(room) do
           {:ok, pid} ->
-            if RoomServer.server_in_room(pid, origin) do
+            if RoomServer.server_in_room?(pid, origin) do
               data = Transaction.new([event])
 
               conn
@@ -57,7 +57,13 @@ defmodule MatrixServerWeb.Federation.EventController do
 
   def state_ids(conn, _), do: put_error(conn, :missing_param)
 
-  @spec get_state_or_state_ids(Plug.Conn.t(), :state | :state_ids, String.t(), String.t(), String.t()) :: Plug.Conn.t()
+  @spec get_state_or_state_ids(
+          Plug.Conn.t(),
+          :state | :state_ids,
+          String.t(),
+          String.t(),
+          String.t()
+        ) :: Plug.Conn.t()
   defp get_state_or_state_ids(conn, state_or_state_ids, origin, event_id, room_id) do
     query =
       Event
@@ -68,7 +74,7 @@ defmodule MatrixServerWeb.Federation.EventController do
       %Event{room: room} = event ->
         case RoomServer.get_room_server(room) do
           {:ok, pid} ->
-            if RoomServer.server_in_room(pid, origin) do
+            if RoomServer.server_in_room?(pid, origin) do
               {state_events, auth_chain} =
                 case state_or_state_ids do
                   :state -> RoomServer.get_state_at_event(pid, event)
diff --git a/lib/matrix_server_web/federation/http_client.ex b/lib/matrix_server_web/federation/http_client.ex
index fff2920..4006665 100644
--- a/lib/matrix_server_web/federation/http_client.ex
+++ b/lib/matrix_server_web/federation/http_client.ex
@@ -75,7 +75,8 @@ defmodule MatrixServerWeb.Federation.HTTPClient do
 
   def get_state_ids(client, room_id, event_id) do
     path =
-      RouteHelpers.event_path(Endpoint, :state_ids, room_id) |> Tesla.build_url(event_id: event_id)
+      RouteHelpers.event_path(Endpoint, :state_ids, room_id)
+      |> Tesla.build_url(event_id: event_id)
 
     Tesla.get(client, path)
   end