This chapter examines the core assumptions of poststructuralism, one of the International Relations (IR) perspectives furthest away from the realist and liberal mainstream. It explores whether language matters for international relations, whether all states have the same identity, and whether the state is the most important actor in world politics today. The chapter also considers poststructuralist views about the social world, state sovereignty, and identity and foreign policy. Finally, it discusses poststructuralism as a political philosophy. Two case studies are presented, one dealing with discourses on the Ebola outbreak in 2014 and the other relating to Russian discourse on Crimea. There is also an Opposing Opinions box that asks whether poststructuralism provides a good account of the role that materiality and power play in world politics.