Simulation games can play a critical role in enabling students to navigate the problem spaces of the past while simultaneously critiquing the models designers offer to represent those problem spaces. There is much to be gained through their use. This includes rich opportunities for students to engage the past as independent historians; to consider choice, cause, and effect; to question and interrogate. During this past decade, there has been a flurry of activity from academics on the power of games to teach modes of thought, skills, and even, to some extent, content knowledge. Too little of that work has been extended into practical guidelines for teachers. In "Gaming the Past," the author offered a detailed set of practical guidelines for conceiving, designing, implementing, and assessing lessons incorporating simulation games. In this article, he intends to elaborate on the theory that makes such lessons valid. Though this article advocates using historical simulation games as learning tools, the purpose is not to give a thorough list of arguments for their use. Rather, the point is to develop more fully what exactly it is that simulations can do as a form of educational media. Supplying this critical link will help focus history educators, from middle school to college, to develop learning environments and resources that leverage the power of simulation games. Perhaps this theory will also provide suggestions for game designers hoping to develop games that have value as interpretations of the past. (Contains 27 notes.)