Competition is commonplace among militant groups. Although political scientists have begun recognizing its importance, they lag behind other fields in the general study of competition. This is critical due to the strategic depth that competition brings. How one group behaves affects another group, and vice versa. Moreover, target governments and international organizations can manipulate the environment in which the groups must then interact. This chapter argues that building models to examine these issues is a useful strategy, but that the literature on political violence has not yet explored the implications. We then set the stage for the results we develop throughout the book.