DOI: 10.1145/1077246.1077255As multiplayer online gaming gains in economic and social importance, an increasingly large number of players is beginning to rely on bots (automated player agents) to gain unfair advantages in games. In this article we study the problem of restricting participation in online games to human players so they can enjoy the game without interference from the bots. We propose two broad approaches to prevent bots from playing online games. The first consists of seamlessly integrating software-based tests (known as reverse Turing tests or CAPTCHA tests) into online games to tell humans and computers apart. Our second contribution is to propose hardware instantiations of CAPTCHA tests. Our techniques are applicable in a wide variety of online games, from poker to "shoot'em ups." They are cost-effective, immune to cheating, and preserve the human players' enjoyment of each game. We conclude with a discussion of how approaches to deter the use of bots may complement our techniques to detect bots.