diff --git a/unobot.py b/unobot.py
index 7ac4895..994aa63 100644
--- a/unobot.py
+++ b/unobot.py
@@ -63,6 +63,9 @@
     'DONT_HAVE':       "You don't have that card!",
     'DOESNT_PLAY':     "That card can't be played now.",
     'UNO':             "UNO! %s has ONE card left!",
+    'CHALLENGE_FALSE': "Challenging failure to call UNO does not make sense at this time.",
+    'CHALLENGE_TRUE':  "%s caught %s forgetting to call UNO! Two penalty cards dealt.",
+    'CHALLENGE_NONE':  "%s holds more than one card. Challenge is void.",
     'WIN':             "We have a winner: %s!!! This game took %s",
     'DRAWN_ALREADY':   "You've already drawn, either play or pass.",
     'DRAWN_CARD':      "You drew: %s",
@@ -128,6 +131,7 @@ def __init__(self, trigger):
         self.way = 1
         self.drawn = NO
         self.smallestHand = DECK_SIZE
+        self.unoCalled = NO
         self.deck = []
         self.startTime = None
 
@@ -249,12 +253,42 @@ def play(self, bot, trigger):
             self.inc_player()
             self.card_played(bot, playcard)
 
-            if len(self.players[self.playerOrder[pl]]) == 1:
-                bot.say(STRINGS['UNO'] % self.playerOrder[pl])
-            elif len(self.players[self.playerOrder[pl]]) == 0:
+            if len(self.players[self.playerOrder[pl]]) == 0:
                 return WIN
             self.show_on_turn(bot)
 
+    def call_uno(self, bot, trigger):
+        if not self.dealt:
+            return
+
+        caller = trigger.nick
+        if caller not in (self.playerOrder[self.currentPlayer], self.playerOrder[self.previousPlayer]):
+            # only the current or previous player can call UNO
+            bot.say(STRINGS['ON_TURN'] % self.playerOrder[self.currentPlayer])
+            return
+        if self.unoCalled == caller:
+            # UNO was already called this turn
+            return
+        self.unoCalled = caller
+        bot.say(STRINGS['UNO'] % caller)
+
+    def challenge_uno(self, bot, trigger):
+        if not self.dealt:
+            return
+
+        prev = self.playerOrder[self.previousPlayer]
+        if len(self.players[prev]) == 1:
+            if self.unoCalled == prev:
+                bot.say(STRINGS['CHALLENGE_FALSE'])
+                return
+            else:
+                bot.say(STRINGS['CHALLENGE_TRUE'] % (trigger.nick, prev))
+                z = [self.get_card(), self.get_card()]
+                self.players[prev].extend(z)
+                bot.notice(STRINGS['DRAWN_CARD'] % self.render_cards(bot, z, prev), prev)
+        else:
+            bot.say(STRINGS['CHALLENGE_NONE'] % prev)
+
     def draw(self, bot, trigger):
         if not self.deck:
             return
@@ -479,6 +513,9 @@ def inc_player(self):
                 self.currentPlayer = 0
             if self.currentPlayer < 0:
                 self.currentPlayer = len(self.players) - 1
+            if self.unoCalled != self.previousPlayer:
+                # Reset once it's too late to challenge a missed UNO call
+                self.unoCalled = NO
 
     def remove_player(self, bot, player):
         if len(self.players) == 1:
@@ -606,6 +643,16 @@ def play(self, bot, trigger):
             bot.say(STRINGS['WIN'] % (winner, game_duration))
             self.game_ended(bot, trigger, winner)
 
+    def call_uno(self, bot, trigger):
+        if trigger.sender not in self.games:
+            return
+        self.games[trigger.sender].call_uno(bot, trigger)
+
+    def challenge_uno(self, bot, trigger):
+        if trigger.sender not in self.games:
+            return
+        self.games[trigger.sender].challenge_uno(bot, trigger)
+
     def draw(self, bot, trigger):
         if trigger.sender not in self.games:
             return
@@ -881,6 +928,20 @@ def unoplay(bot, trigger):
     unobot.play(bot, trigger)
 
 
+@module.rule('^uno!?$')
+@module.priority('high')
+@module.require_chanmsg
+def unocall(bot, trigger):
+    unobot.call_uno(bot, trigger)
+
+
+@module.commands('unofail')
+@module.priority('medium')
+@module.require_chanmsg
+def unofail(bot, trigger):
+    unobot.challenge_uno(bot, trigger)
+
+
 @module.commands('draw')
 @module.priority('medium')
 @module.require_chanmsg