This chapter examines the impact of social constructivism on Security Studies, and how it calls into question the assumed orthodoxy of rationalist approaches to security and the international system by asking how security and security threats are ‘socially constructed’. It focuses on the importance of social relations and why identity, norms, and culture matter. Whereas rationalist approaches focus on material forces to understand and theorize security, social constructivism argues that ideational as well as material factors construct the world around us and the meanings we give to it. Therefore, its significance for Security Studies is crucial in terms not only of conceptualizing security but of providing alternative readings of security. However, constructivism is not a uniform approach. As this chapter demonstrates, it is broadly divided into two camps, which differ on questions of methodology and particular aspects of how knowledge and identity are interrogated. Throughout this chapter case studies of constructivist approaches to security questions will be discussed, and the chapter concludes with a consideration of critiques of constructivism.