In Emanuel Adler's distinctive constructivist approach to international relations theory, international practices evolve in tandem with collective knowledge of the material and social worlds. This book - comprising a fresh selection of his journal publications, a substantial new introduction, three previously unpublished articles - points IR constructivism in a novel direction, characterized as 'communitarian'. Adler's synthesis does not herald the end of the nation-state; nor does it suggest that agency is unimportant in international life. Rather, it argues that what mediates between individual and state agency and social structures are communities of practice, which are the wellspring and repositories of collective meanings and social practices. The concept of communities of practice casts new light on epistemic communities and security communities, helping to explain why certain ideas congeal into human practices and others do not, and which social mechanisms can facilitate the emergence of normatively better communities.