Captured by Alexander Wendt’s now-famous maxim “anarchy is what states make of it,” social constructivism is the idea that the world “out there” is not given, as realists would argue, but rather, “socially constructed.” In doing so, social constructivism places a focus on the importance of “mutual constitution”: international politics is shaped by both structures, such as anarchy, or agents, such as states and other actors. Structures and agents influence each other. Moreover, social constructivism emphasizes social relations in global politics, and sees security and international politics as determined by ideas as well as material factors. The identity of agents such as states matter because identity helps determine national interests. As states interact with other actors in the international system, their ideas and identity can change over time, which can produce a more dynamic understanding of international relations. This chapter will take the reader through the key ideas of social constructivism – also referred to as “constructivism” in this chapter – showing how norms, culture, and ideas about identity shape actors, condition their relations with each other, and can impact the so-called “given” nature of international relations and transform understandings of power relations. Social constructivism can also help make sense of security and military phenomena, such as alliances and threat perceptions, or why states go to war. This chapter will also cover the different branches of constructivist thought and the main critiques of constructivism to highlight its key contributions and the problems it also raises.