This chapter introduces postcolonialism as a set of increasingly influential positions and perspectives within the wider discipline of International Relations and sketches its implications for security studies. It begins with postcolonialism’s genealogies, tracing its emergence in a set of transnational debates about the mutually constitutive relations between knowledge and imperialism. The chapter then lays out the standard account of world history as organized around Westphalian sovereignty which informs security studies and shows how postcolonialism puts it in question, forcing the international to be reconceived as the context within which security is defined, practised, and studied. Third, the chapter puts postcolonialism to work and discusses what it might mean to decolonize security studies. In a short conclusion, it returns to the question of the tense relations between security studies and postcolonialism itself.