The scientific study of serious games is a recent development, spanning less than two decades. One aspect in this field is human computation with digital games. The core of the paradigm is to outsource problems that are not yet solvable by conventional computational systems to humans. Therefore, these problems are reformulated into tasks that are then integrated into digital games. The players of the game then solve the problem while playing. Different human computation games have been successfully deployed, but tend to provide a relatively narrow gaming experience. This chapter analyzes the differences between game design for human computation and traditional digital game design. An in-depth consideration of these differences shows that it is a viable approach to build human computation games with a wide range of designs. The key issues of human computation game design are illustrated with the game OnToGalaxy.