This chapter examines the impact of social constructivism on Security Studies as well as its critique of the assumed orthodoxy of rationalist approaches to security and the international system. In particular, it considers the manner that social constructivists address the question of how security and security threats are ‘socially constructed’. The chapter first provides an overview of definitions and key concepts relating to constructivism, such as its emphasis on the importance of ideas, identity, and interaction, along with its alternative approach to thinking about security. It then explores Alexander Wendt’s three cultures of anarchy and compares conventional constructivism with critical constructivism. Finally, it analyses rationalist and poststructuralist critiques of constructivism.