diff --git a/src/kike.c b/src/kike.c
index d3415e7..cdb20ca 100644
--- a/src/kike.c
+++ b/src/kike.c
@@ -292,20 +292,24 @@ client_free (struct client *self)
 	free (self->away_message);
 }
 
+static void
+client_mode_to_str (unsigned m, struct str *out)
+{
+	if (m & IRC_USER_MODE_INVISIBLE)           str_append_c (out, 'i');
+	if (m & IRC_USER_MODE_RX_WALLOPS)          str_append_c (out, 'w');
+	if (m & IRC_USER_MODE_RESTRICTED)          str_append_c (out, 'r');
+	if (m & IRC_USER_MODE_OPERATOR)            str_append_c (out, 'o');
+	if (m & IRC_USER_MODE_RX_SERVER_NOTICES)   str_append_c (out, 's');
+}
+
 static char *
 client_get_mode (struct client *self)
 {
 	struct str mode;
 	str_init (&mode);
-
-	if (self->away_message)                    str_append_c (&mode, 'a');
-
-	unsigned m = self->mode;
-	if (m & IRC_USER_MODE_INVISIBLE)           str_append_c (&mode, 'i');
-	if (m & IRC_USER_MODE_RX_WALLOPS)          str_append_c (&mode, 'w');
-	if (m & IRC_USER_MODE_RESTRICTED)          str_append_c (&mode, 'r');
-	if (m & IRC_USER_MODE_OPERATOR)            str_append_c (&mode, 'o');
-	if (m & IRC_USER_MODE_RX_SERVER_NOTICES)   str_append_c (&mode, 's');
+	if (self->away_message)
+		str_append_c (&mode, 'a');
+	client_mode_to_str (self->mode, &mode);
 	return str_steal (&mode);
 }
 
@@ -887,6 +891,7 @@ enum
 	IRC_ERR_NOPRIVILEGES          = 481,
 	IRC_ERR_CHANOPRIVSNEEDED      = 482,
 
+	IRC_ERR_UMODEUNKNOWNFLAG      = 501,
 	IRC_ERR_USERSDONTMATCH        = 502
 };
 
@@ -963,6 +968,7 @@ static const char *g_default_replies[] =
 	[IRC_ERR_NOPRIVILEGES] = ":Permission Denied- You're not an IRC operator",
 	[IRC_ERR_CHANOPRIVSNEEDED] = "%s :You're not channel operator",
 
+	[IRC_ERR_UMODEUNKNOWNFLAG] = ":Unknown MODE flag",
 	[IRC_ERR_USERSDONTMATCH] = ":Cannot change mode for other users",
 };
 
@@ -1295,6 +1301,83 @@ irc_maybe_send_channel_list (struct client *c, struct channel *chan,
 	return true;
 }
 
+static void
+irc_modify_mode (unsigned *mask, unsigned mode, bool add)
+{
+	if (add)
+		*mask |= mode;
+	else
+		*mask &= ~mode;
+}
+
+static void
+irc_update_user_mode (struct client *c, unsigned new_mode)
+{
+	unsigned old_mode = c->mode;
+	c->mode = new_mode;
+
+	unsigned added   = new_mode & ~old_mode;
+	unsigned removed = old_mode & ~new_mode;
+
+	struct str diff;
+	str_init (&diff);
+
+	if (added)
+	{
+		str_append_c (&diff, '+');
+		client_mode_to_str (added, &diff);
+	}
+	if (removed)
+	{
+		str_append_c (&diff, '-');
+		client_mode_to_str (removed, &diff);
+	}
+
+	if (diff.len)
+		irc_send (c, ":%s MODE %s :%s",
+			c->nickname, c->nickname, diff.str);
+	str_free (&diff);
+}
+
+static void
+irc_handle_user_mode_change (struct client *c, const char *mode_string)
+{
+	unsigned new_mode = c->mode;
+	bool adding = true;
+
+	while (*mode_string)
+	switch (*mode_string++)
+	{
+	case '+':  adding = true;   break;
+	case '-':  adding = false;  break;
+
+	case 'a':
+		// Ignore, the client should use AWAY
+		break;
+	case 'i':
+		irc_modify_mode (&new_mode, IRC_USER_MODE_INVISIBLE, adding);
+		break;
+	case 'w':
+		irc_modify_mode (&new_mode, IRC_USER_MODE_RX_WALLOPS, adding);
+		break;
+	case 'r':
+		if (adding)
+			irc_modify_mode (&new_mode, IRC_USER_MODE_RESTRICTED, true);
+		break;
+	case 'o':
+		if (!adding)
+			irc_modify_mode (&new_mode, IRC_USER_MODE_OPERATOR, false);
+		// TODO: check public key fingerprint when adding
+		break;
+	case 's':
+		irc_modify_mode (&new_mode, IRC_USER_MODE_RX_SERVER_NOTICES, adding);
+		break;
+	default:
+		RETURN_WITH_REPLY (c, IRC_ERR_UMODEUNKNOWNFLAG);
+	}
+	irc_update_user_mode (c, new_mode);
+}
+
 static void
 irc_handle_mode (const struct irc_message *msg, struct client *c)
 {
@@ -1308,11 +1391,14 @@ irc_handle_mode (const struct irc_message *msg, struct client *c)
 		if (irc_strcmp (target, c->nickname))
 			RETURN_WITH_REPLY (c, IRC_ERR_USERSDONTMATCH);
 
-		char *mode = client_get_mode (client);
-		irc_send_reply (c, IRC_RPL_UMODEIS, mode);
-		free (mode);
-
-		// TODO: mode modification
+		if (msg->params.len < 2)
+		{
+			char *mode = client_get_mode (client);
+			irc_send_reply (c, IRC_RPL_UMODEIS, mode);
+			free (mode);
+		}
+		else
+			irc_handle_user_mode_change (c, msg->params.vector[1]);
 		return;
 	}