diff --git a/lib/matrix_server/event.ex b/lib/matrix_server/event.ex
index 032c8bc..739322a 100644
--- a/lib/matrix_server/event.ex
+++ b/lib/matrix_server/event.ex
@@ -229,7 +229,6 @@ defmodule MatrixServer.Event do
        when sender != state_key,
        do: false
 
-  # TODO: Rule 5.3.1
   # All other rules will be checked during state resolution.
   defp do_prevalidate(_, _, _), do: true
 
diff --git a/lib/matrix_server/state_resolution.ex b/lib/matrix_server/state_resolution.ex
index 4488e06..0528497 100644
--- a/lib/matrix_server/state_resolution.ex
+++ b/lib/matrix_server/state_resolution.ex
@@ -35,9 +35,309 @@ defmodule MatrixServer.StateResolution do
     end
   end
 
-  def do_resolve([], _), do: %{}
+  def is_authorized_by_auth_events(%Event{auth_events: auth_event_ids} = event) do
+    # We assume the auth events are validated beforehand.
+    state_set =
+      Event
+      |> where([e], e.event_id in ^auth_event_ids)
+      |> Repo.all()
+      |> Enum.reduce(%{}, &update_state_set/2)
 
-  def do_resolve(state_sets, room_events) do
+    is_authorized(event, state_set)
+  end
+
+  def resolve_forward_extremities(%Event{room_id: room_id}) do
+    room_events =
+      Event
+      |> where([e], e.room_id == ^room_id)
+      |> select([e], {e.event_id, e})
+      |> Repo.all()
+      |> Enum.into(%{})
+
+    Event
+    |> where([e], e.room_id == ^room_id)
+    |> join(:inner, [e], r in Room, on: e.room_id == r.id)
+    |> where([e, r], e.event_id == fragment("ANY(?)", r.forward_extremities))
+    |> Repo.all()
+    |> Enum.map(&resolve/1)
+    |> do_resolve(room_events)
+  end
+
+  def is_authorized(%Event{type: "m.room.create", prev_events: prev_events}, %{}),
+    do: prev_events == []
+
+  # Check rule: 5.2.1
+  def is_authorized(%Event{type: "m.room.member", state_key: state_key}, %{
+        {"m.room.create", ""} => %Event{content: %{"creator" => creator}}
+      }),
+      do: state_key == creator
+
+  def is_authorized(
+        %Event{type: "m.room.member", sender: sender, content: %{"membership" => "join"}},
+        state_set
+      ) do
+    join_rule = get_join_rule(state_set)
+    membership = get_membership(sender, state_set)
+
+    # Check rules: 5.2.3, 5.2.4, 5.2.5
+    cond do
+      membership == "ban" -> false
+      join_rule == "invite" -> membership in ["invite", "join"]
+      join_rule == "public" -> true
+      true -> false
+    end
+  end
+
+  # TODO: rule 5.3.1
+  def is_authorized(
+        %Event{
+          type: "m.room.member",
+          content: %{"membership" => "invite", "third_party_invite" => _}
+        },
+        _
+      ),
+      do: false
+
+  def is_authorized(
+        %Event{
+          type: "m.room.member",
+          sender: sender,
+          content: %{"membership" => "invite"},
+          state_key: state_key
+        },
+        state_set
+      ) do
+    sender_membership = get_membership(sender, state_set)
+    target_membership = get_membership(state_key, state_set)
+    power_levels = get_power_levels(state_set)
+
+    # Check rules: 5.3.2, 5.3.3, 5.3.4
+    cond do
+      sender_membership != "join" -> false
+      target_membership in ["join", "ban"] -> false
+      has_power_level(sender, power_levels, :invite) -> true
+      true -> false
+    end
+  end
+
+  def is_authorized(
+        %Event{
+          type: "m.room.member",
+          sender: sender,
+          content: %{"membership" => "leave"},
+          state_key: sender
+        },
+        state_set
+      ) do
+    # Check rule: 5.4.1
+    get_membership(sender, state_set) in ["invite", "join"]
+  end
+
+  def is_authorized(
+        %Event{
+          type: "m.room.member",
+          sender: sender,
+          content: %{"membership" => "leave"},
+          state_key: state_key
+        },
+        state_set
+      ) do
+    sender_membership = get_membership(sender, state_set)
+    target_membership = get_membership(state_key, state_set)
+    power_levels = get_power_levels(state_set)
+    sender_pl = get_user_power_level(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
+      target_membership == "ban" and not has_power_level(sender, power_levels, :ban) -> false
+      has_power_level(sender, power_levels, :kick) and target_pl < sender_pl -> true
+      true -> false
+    end
+  end
+
+  def is_authorized(
+        %Event{
+          type: "m.room.member",
+          sender: sender,
+          content: %{"membership" => "ban"},
+          state_key: state_key
+        },
+        state_set
+      ) do
+    sender_membership = get_membership(sender, state_set)
+    power_levels = get_power_levels(state_set)
+    sender_pl = get_user_power_level(sender, power_levels)
+    target_pl = get_user_power_level(state_key, power_levels)
+
+    # Check rules: 5.5.1, 5.5.2
+    cond do
+      sender_membership != "join" -> false
+      has_power_level(sender, power_levels, :ban) and target_pl < sender_pl -> true
+      true -> false
+    end
+  end
+
+  # Check rule: 5.6
+  def is_authorized(%Event{type: "m.room.member"}, _), do: false
+
+  def is_authorized(%Event{sender: sender} = event, state_set) do
+    # Check rule: 6
+    get_membership(sender, state_set) == "join" and _is_authorized(event, state_set)
+  end
+
+  defp _is_authorized(%Event{type: "m.room.third_party_invite", sender: sender}, state_set) do
+    # Check rule: 7.1
+    has_power_level(sender, state_set, :invite)
+  end
+
+  defp _is_authorized(%Event{state_key: state_key, sender: sender} = event, state_set) do
+    power_levels = get_power_levels(state_set)
+
+    # Check rules: 8, 9
+    cond do
+      not has_power_level(sender, power_levels, {:event, event}) -> false
+      String.starts_with?(state_key, "@") and state_key != sender -> false
+      true -> __is_authorized(event, state_set)
+    end
+  end
+
+  defp __is_authorized(
+         %Event{type: "m.room.power_levels", sender: sender, content: content},
+         state_set
+       ) do
+    current_pls = get_power_levels(state_set)
+    new_pls = content
+    sender_pl = get_user_power_level(sender, new_pls)
+
+    # Check rules: 10.2, 10.3, 10.4, 10.5
+    cond do
+      not is_map_key(state_set, {"m.room.power_levels", ""}) -> true
+      not authorize_power_levels(sender, sender_pl, current_pls, new_pls) -> false
+      true -> true
+    end
+  end
+
+  # TODO: Rule 11
+
+  defp __is_authorized(_, _), do: true
+
+  defp authorize_power_levels(
+         user,
+         user_pl,
+         %{"events" => current_events, "users" => current_users} = current_pls,
+         %{"events" => new_events, "users" => new_users} = new_pls
+       ) do
+    keys = ["users_default", "events_default", "state_default", "ban", "redact", "kick", "invite"]
+
+    valid_power_level_key_changes(Map.take(current_pls, keys), Map.take(new_pls, keys), user_pl) and
+      valid_power_level_key_changes(current_events, new_events, user_pl) and
+      valid_power_level_key_changes(current_users, new_users, user_pl) and
+      valid_power_level_users_changes(current_users, new_users, user, user_pl)
+  end
+
+  defp has_power_level(user, power_levels, action) do
+    user_pl = get_user_power_level(user, power_levels)
+    action_pl = get_action_power_level(action, power_levels)
+
+    user_pl >= action_pl
+  end
+
+  defp get_user_power_level(user, %{"users" => users}) when is_map_key(users, user),
+    do: users[user]
+
+  defp get_user_power_level(_, %{"users_default" => pl}), do: pl
+  defp get_user_power_level(_, _), do: 0
+
+  defp get_action_power_level(:invite, %{"invite" => pl}), do: pl
+  defp get_action_power_level(:invite, _), do: 50
+  defp get_action_power_level(:ban, %{"ban" => pl}), do: pl
+  defp get_action_power_level(:ban, _), do: 50
+  defp get_action_power_level(:redact, %{"redact" => pl}), do: pl
+  defp get_action_power_level(:redact, _), do: 50
+
+  defp get_action_power_level({:event, %Event{type: type}}, %{"events" => events})
+       when is_map_key(events, type),
+       do: events[type]
+
+  defp get_action_power_level({:event, event}, power_levels) do
+    if Event.is_state_event(event) do
+      case power_levels do
+        %{"state_default" => pl} -> pl
+        %{} -> 50
+        _ -> 0
+      end
+    else
+      case power_levels do
+        %{"events_default" => pl} -> pl
+        _ -> 0
+      end
+    end
+  end
+
+  defp get_power_levels(state_set) do
+    case state_set[{"m.room.power_levels", ""}] do
+      %Event{content: content} -> content
+      nil -> nil
+    end
+  end
+
+  defp get_join_rule(state_set) do
+    case state_set[{"m.room.join_rules", ""}] do
+      %Event{content: %{"join_rule" => join_rule}} -> join_rule
+      nil -> nil
+    end
+  end
+
+  defp get_membership(user, state_set) do
+    case state_set[{"m.room.member", user}] do
+      %Event{content: %{"membership" => membership}} -> membership
+      nil -> nil
+    end
+  end
+
+  defp valid_power_level_key_changes(l1, l2, user_pl) do
+    set1 = MapSet.new(l1)
+    set2 = MapSet.new(l2)
+
+    MapSet.difference(
+      MapSet.union(set1, set2),
+      MapSet.intersection(set1, set2)
+    )
+    |> Enum.group_by(&elem(&1, 0), &elem(&1, 1))
+    |> Enum.all?(fn {_k, values} ->
+      Enum.all?(values, &(&1 <= user_pl))
+    end)
+  end
+
+  defp valid_power_level_users_changes(current_users, new_users, user, user_pl) do
+    set1 = MapSet.new(current_users)
+    set2 = MapSet.new(new_users)
+
+    MapSet.difference(
+      MapSet.union(set1, set2),
+      MapSet.intersection(set1, set2)
+    )
+    |> Enum.all?(fn
+      {_k, values} when length(values) != 2 -> true
+      {k, _} when k == user -> true
+      {_k, [old_value, _]} -> old_value != user_pl
+    end)
+  end
+
+  def testing do
+    %Event{content: content} = event = Event.power_levels("room1", "charlie")
+    event = %Event{event | content: %{content | "ban" => 0}}
+
+    event
+    |> Map.put(:prev_events, ["b", "fork"])
+    |> Map.put(:auth_events, ["create", "join_charlie", "b"])
+  end
+
+  defp do_resolve([], _), do: %{}
+
+  defp do_resolve(state_sets, room_events) do
     {unconflicted_state_map, conflicted_state_set} = calculate_conflict(state_sets, room_events)
 
     if MapSet.size(conflicted_state_set) == 0 do
@@ -47,7 +347,7 @@ defmodule MatrixServer.StateResolution do
     end
   end
 
-  def do_resolve(state_sets, room_events, unconflicted_state_map, conflicted_state_set) do
+  defp do_resolve(state_sets, room_events, unconflicted_state_map, conflicted_state_set) do
     full_conflicted_set =
       MapSet.union(conflicted_state_set, auth_difference(state_sets, room_events))
 
@@ -81,7 +381,7 @@ defmodule MatrixServer.StateResolution do
     |> Map.merge(unconflicted_state_map)
   end
 
-  def calculate_conflict(state_sets, room_events) do
+  defp calculate_conflict(state_sets, room_events) do
     {unconflicted, conflicted} =
       state_sets
       |> Enum.flat_map(&Map.keys/1)
@@ -116,7 +416,7 @@ defmodule MatrixServer.StateResolution do
     {unconflicted_state_map, conflicted_state_set}
   end
 
-  def auth_difference(state_sets, room_events) do
+  defp auth_difference(state_sets, room_events) do
     full_auth_chains =
       Enum.map(state_sets, fn state_set ->
         state_set
@@ -130,13 +430,13 @@ defmodule MatrixServer.StateResolution do
     MapSet.difference(auth_chain_union, auth_chain_intersection)
   end
 
-  def full_auth_chain(events, room_events) do
+  defp full_auth_chain(events, room_events) do
     events
     |> Enum.map(&auth_chain(&1, room_events))
     |> Enum.reduce(MapSet.new(), &MapSet.union/2)
   end
 
-  def auth_chain(%Event{auth_events: auth_events}, room_events) do
+  defp auth_chain(%Event{auth_events: auth_events}, room_events) do
     auth_events
     |> Enum.map(&room_events[&1])
     |> Enum.reduce(MapSet.new(), fn %Event{event_id: auth_event_id} = auth_event, acc ->
@@ -147,7 +447,7 @@ defmodule MatrixServer.StateResolution do
     end)
   end
 
-  def rev_top_pow_order(room_events) do
+  defp rev_top_pow_order(room_events) do
     fn %Event{origin_server_ts: timestamp1, event_id: event_id1} = event1,
        %Event{origin_server_ts: timestamp2, event_id: event_id2} = event2 ->
       power1 = get_power_level(event1, room_events)
@@ -165,19 +465,20 @@ defmodule MatrixServer.StateResolution do
     end
   end
 
-  def get_power_level(%Event{sender: sender, auth_events: auth_event_ids}, room_events) do
+  defp get_power_level(%Event{sender: sender, auth_events: auth_event_ids}, room_events) do
     pl_event_id =
       Enum.find(auth_event_ids, fn id ->
         room_events[id].type == "m.room.power_levels"
       end)
 
+    # TODO: refactor
     case room_events[pl_event_id] do
       %Event{content: %{"users" => pl_users}} -> Map.get(pl_users, sender, 0)
       nil -> 0
     end
   end
 
-  def mainline_order(event, room_events) do
+  defp mainline_order(event, room_events) do
     mainline_map =
       event
       |> mainline(room_events)
@@ -220,13 +521,13 @@ defmodule MatrixServer.StateResolution do
     end
   end
 
-  def mainline(event, room_events) do
+  defp mainline(event, room_events) do
     event
     |> mainline([], room_events)
     |> Enum.reverse()
   end
 
-  def mainline(%Event{auth_events: auth_event_ids} = event, acc, room_events) do
+  defp mainline(%Event{auth_events: auth_event_ids} = event, acc, room_events) do
     pl_event_id =
       Enum.find(auth_event_ids, fn id ->
         room_events[id].type == "m.room.power_levels"
@@ -238,20 +539,20 @@ defmodule MatrixServer.StateResolution do
     end
   end
 
-  def iterative_auth_checks(events, state_set, room_events) do
+  defp iterative_auth_checks(events, state_set, room_events) do
     Enum.reduce(events, state_set, fn event, acc ->
       if is_authorized2(event, acc, room_events), do: update_state_set(event, acc), else: acc
     end)
   end
 
-  def update_state_set(
-        %Event{type: event_type, state_key: state_key} = event,
-        state_set
-      ) do
+  defp update_state_set(
+         %Event{type: event_type, state_key: state_key} = event,
+         state_set
+       ) do
     Map.put(state_set, {event_type, state_key}, event)
   end
 
-  def is_authorized2(%Event{auth_events: auth_event_ids} = event, state_set, room_events) do
+  defp is_authorized2(%Event{auth_events: auth_event_ids} = event, state_set, room_events) do
     state_set =
       auth_event_ids
       |> Enum.map(&room_events[&1])
@@ -259,94 +560,4 @@ defmodule MatrixServer.StateResolution do
 
     is_authorized(event, state_set)
   end
-
-  # TODO: join and power levels events
-  def is_authorized(%Event{type: "m.room.create", prev_events: prev_events}, _),
-    do: prev_events == []
-
-  def is_authorized(
-        %Event{type: "m.room.member", content: %{"membership" => "join"}, state_key: user},
-        state_set
-      ) do
-    allowed_to_join(user, state_set)
-  end
-
-  def is_authorized(%Event{sender: sender} = event, state_set) do
-    in_room(sender, state_set) and
-      has_power_level(
-        sender,
-        get_power_levels(state_set),
-        get_event_power_level(event)
-      )
-  end
-
-  def in_room(user, state_set) when is_map_key(state_set, {"m.room.member", user}) do
-    case state_set[{"m.room.member", user}].content["membership"] do
-      "join" -> true
-      _ -> false
-    end
-  end
-
-  def in_room(_, _), do: false
-
-  def get_power_levels(state_set)
-      when is_map_key(state_set, {"m.room.power_levels", ""}) do
-    state_set[{"m.room.power_levels", ""}].content
-  end
-
-  def get_power_levels(_), do: nil
-
-  def has_power_level(user, %{"users" => users}, level) do
-    Map.get(users, user, 0) >= level
-  end
-
-  def has_power_level(_, _, _) do
-    true
-  end
-
-  defp get_event_power_level(%Event{state_key: ""}), do: 0
-  defp get_event_power_level(_), do: 50
-
-  # No join rules specified, allow joining for room creator only.
-  def allowed_to_join(user, state_set)
-      when not is_map_key(state_set, {"m.room.join_rules", ""}) do
-    state_set[{"m.room.create", ""}].sender == user
-  end
-
-  def is_authorized_by_auth_events(%Event{auth_events: auth_event_ids} = event) do
-    # We assume the auth events are validated beforehand.
-    state_set =
-      Event
-      |> where([e], e.event_id in ^auth_event_ids)
-      |> Repo.all()
-      |> Enum.reduce(%{}, &update_state_set/2)
-
-    is_authorized(event, state_set)
-  end
-
-  def resolve_forward_extremities(%Event{room_id: room_id}) do
-    room_events =
-      Event
-      |> where([e], e.room_id == ^room_id)
-      |> select([e], {e.event_id, e})
-      |> Repo.all()
-      |> Enum.into(%{})
-
-    Event
-    |> where([e], e.room_id == ^room_id)
-    |> join(:inner, [e], r in Room, on: e.room_id == r.id)
-    |> where([e, r], e.event_id == fragment("ANY(?)", r.forward_extremities))
-    |> Repo.all()
-    |> Enum.map(&resolve/1)
-    |> do_resolve(room_events)
-  end
-
-  def testing do
-    %Event{content: content} = event = Event.power_levels("room1", "charlie")
-    event = %Event{event | content: %{content | "ban" => 0}}
-
-    event
-    |> Map.put(:prev_events, ["b", "fork"])
-    |> Map.put(:auth_events, ["create", "join_charlie", "b"])
-  end
 end