International relations scholars have tended to focus on realism's common features rather than exploring potential differences. Realists do share certain assumptions and are often treated as a group, but such a broad grouping obscures systematic divisions within realist theory. Recently, some analysts have argued that it is necessary to differentiate within realism. This article builds on this line of argument. The potential, and need, to divide realism on the basis of divergent assumptions has so far been overlooked. In this article I argue that realism can be split into two competing branches by revealing latent divisions regarding a series of assumptions about state behavior. The first branch is Kenneth Waltz's well-known neorealist theory; a second branch, termed here “postclassical realism,” has yet to be delineated as a major alternative but corresponds with a number of realist analyses that cohere with one another and are incompatible with Waltzian neorealism.