In this chapter, students will learn about constructivist approaches to security. Constructivism has become an increasingly prominent theoretical approach to international relations since its emergence in the 1980s. Focusing on the role of ideational factors and the social construction of world politics, it is perhaps best described as a broader social theory, which then informs how we might approach the study of security. This chapter draws out key contributions of constructivist thought that have been applied to security studies. It introduces students to the idea of security as socially constructed, before examining constructivist concerns with ideational factors such as norms and identity. The chapter then outlines what can be gained through understanding security as the product of processes of negotiation and contestation, and by approaching the relationship between agents and structures in the international system as mutually constitutive.