Tic-tac-toe (or Noughts and crosses, Xs and Os) is a pencil-and-paper game for two players, X and O, who take turns marking the spaces in a 33 grid. The player who succeeds in placing three respective marks in a horizontal, vertical, or diagonal row wins the game.

Players soon discover that best play from both parties leads to a draw (often referred to as cat or cat's game). Hence, Tic-tac-toe is most often played by young children.

The friendliness of Tic-tac-toe games makes them ideal as a pedagogical tool for teaching the concepts of good sportsmanship and the branch of artificial intelligence that deals with the searching of game trees. It is straightforward to write a computer program to play Tic-tac-toe perfectly, to enumerate the 765 essentially different positions (the state space complexity), or the 26,830 possible games up to rotations and reflections (the game tree complexity) on this space.