This chapter lays the foundation for our study of game-playing AI, providing a taxonomy of roles and uses. Two key distinctions are whether the AI plays in the role of a player, or in the role of the non-player character, and whether AI is playing to win or to elicit an experience. Further dimensions of the taxonomy focus on characteristics of the game, and of the interface between the algorithm and the game. The taxonomy given here helps make sense of the very different use cases for AI game playing, and explains why different AI game-playing problems require very different methods.