This chapter examines the key debates that have shaped the development of constructivism in International Relations (IR). It first considers the idea that international relations is a social construction, as it emerged from the critique of more traditional theories of IR. It then explores the distinctions among various constructivisms, with particular emphasis on the contrast between those who seek a ‘better’ social science, and hence better theory, versus those who argue that constructivism is an approach that rests on assumptions at odds with those of positivist method. The chapter proceeds by discussing constructivists’ critique of rationalism, along with constructivism as a ‘middle ground’ between rationalist and poststructuralist approaches to IR. It also analyses the role of language and causality in the debate between rationalists and constructivists. Finally, it links all these insights to the War on Terror and the war on Covid-19.