diff --git a/ttt/game.py b/ttt/game.py
index 2f0e302..68983af 100644
--- a/ttt/game.py
+++ b/ttt/game.py
@@ -58,4 +58,15 @@ class TTTGame:
 
     def check_winner(self, state, symbol):
         "Checks whether the player with `symbol` has won the game."
+        b = state["board"]
+        for r in range(3):
+            if b[3*r] == symbol and b[3*r+1] == symbol and b[3*r+2] == symbol:
+                return True
+        for c in range(3):
+            if b[c] == symbol and b[c+3] == symbol and b[c+6] == symbol:
+                return True
+        if b[0] == symbol and b[4] == symbol and b[8] == symbol:
+            return True
+        if b[2] == symbol and b[4] == symbol and b[6] == symbol:
+            return True
         return False