This chapter discusses the concept of polities, a capacity to mobilize resources, and a degree of institutionalization and hierarchy. It gives a précis of International Relations (IR) work on how world-historical polities have become so-called like units. Given that IR’s remit in the division of labor between the social sciences is to look at generalized social and political relations and interstices between different kinds of groups, polities have been a key site of work. Work has focused on Europe and Europe’s colonial relations with the rest of Europe from the Renaissance. There are vibrant discussions in the field regarding polities in world history. Works discussing relations between European and other polities in a noncolonial context have begun to emerge. The chapter finally discusses characteristic general traits of the three categories of polities that are of major interest for IR today: states, empires and, residually, “nonstate” actors.