This chapter introduces the principal actor in international relations: the sovereign state. First, it defines the state. Second, it explores the origins of the state in the transition from the medieval to the modern world. Third, it examines the concept of sovereignty, especially as it was articulated in early modern political thought. Fourth, it surveys different historical explanations of how the sovereign state triumphed over alternative forms of political society. Finally, it outlines some of the continuing debates about the modern state.