New lines of theorizing in international relations don’t appear very often. Realism proudly proclaims a lineage of 2,500-odd years. Liberalism, in its various forms, traces its roots back several centuries. The appearance and spread of constructivism in the 1990s thus invite explanation. In this essay we explore the “construction of constructivism”—both the conditions of possibility for intellectual change and the goals of scholars proposing it. Constructivism’s success was both unexpected and, in some ways, unintended. Proponents of existing theories remained (and remain) confident in their own tools and early constructivists often had modest goals. Constructivism took off for at least two broad reasons. One was the intellectual landscape of the late 1980s and early 1990s. This was a period when IR theory in the United States had sharply narrowed its concerns, leaving mainstream scholarship on the back foot when the USSR collapsed, and the Cold War ended. The second set of reasons lay in the nature of the constructivist ideas, themselves. Intellectually, constructivist ideas had a plasticity and capaciousness that other IR theories did not. Theoretically, it is a social theory, not specific to IR, which made it useful for tackling a broad range of problems. Empirically, it was portable and open to political analysis at all levels and in all places. Methodologically, it was pluralistic; scholars can and have used diverse methods to explore its claims. In this situation, constructivist scholars, overwhelmingly young and untenured, worked hard to carve out a niche for themselves in the field. That these ideas caught on and became broadly popular has surprised us as much as anyone.