This chapter reflects on the tradition of Western political thought known as realism. Its main purpose is to identify who realists are, and to explain what realism is in the study of international relations. The first part of the chapter introduces students to some important thinkers, both ancient and modern, ascribed to the realist tradition. It also identifies two broad strands of realist thought: ‘classical’ and ‘structural’ or ‘neorealist’. The second part investigates attempts to conceive realism as a unified theory and practice of international relations. It highlights realism’s central concepts of the state and anarchy before reflecting on realism’s normative dimension.