This chapter examines the basic assumptions and foreign policy relevance of constructivism. Using European security as an illustrative example, it shows that constructivism is a valuable tool not only for understanding foreign policy, but also as a guide for prescribing foreign policy. The chapter first explains what constructivism is, outlining the constructivist view that anarchy exists in different forms with major implications for how agents act. It then considers some of the main propositions and conceptual tools of constructivism, with particular emphasis on its arguments regarding identity, social construction, rules, and practice. It also analyses constructivism’s alternative understandings of NATO’s role after the end of the Cold War and in present-day European security. It asserts that theory is important in foreign policy making — including constructivism — because different theories imply different policies and may make alternative policy options visible which would otherwise easily have been overlooked.