Over the last thirty or so years, international law and legal practice have become increasingly more specialized and diversified. These developments come with an increasingly divergent legal practice, in what has been coined as 'special regimes'. This book proposes a new understanding of the concept of a special regime to explain why specialists in different fields of international law do similar things differently. It argues that special regimes are best conceived as communities of practice, in the sense of Etienne Wenger's theory of communities of practice. It explores how the theory of communities of practice translates to the context of international law and the concept of a special regime. The authors draw up an innovative methodology to investigate their theory, focused on the conduct of community members, and apply this method to selected case studies, offering an original approach to the understanding of the special regimes in international law.