In recent years, a great deal has been written about a `constructivist' approach in International Relations, which argues that international reality is socially constructed by cognitive structures that give meaning to the material world. Nevertheless, most of the epistemological, theoretical, empirical and methodological foundations of constructivism remain unclear. Nor are its potential contributions to a better understanding of International Relations widely appreciated. The present article seeks to fill some of these gaps. Constructivism occupies the middle ground between rationalist approaches (whether realist or liberal) and interpretive approaches (mainly postmodernist, poststructuralist and critical), and creates new areas for theoretical and empirical investigation. The bulk of the article lays out the social-epistemological basis of the constructivist approach; juxtaposes constructivism to rationalism and poststructuralism and explains its advantages; presents the concept of cognitive evolution as a way of explaining the social construction of reality; and suggests ways of expanding constructivist research agendas.