This major extension of existing scholarship on the fragmentation of international law utilises the concept of ‘regimes’ from international law and international relations literature to define functional areas of legal specialisation and institutional activity. Responding to prevailing approaches, which focus on the resolution of conflicting norms between regimes, it explores the way in which norms and institutions from disparate regimes overlap and interact. Leading scholars reflect on how, in situations of legal pluralism and concurrent activity, such interaction shapes and controls knowledge and norms at the domestic, transnational and international level, in often hegemonic ways. The contributors draw on topical examples of interacting regimes, including climate, trade, intellectual property and investment regimes, to argue for new methods and understanding of regime interaction. Together, the essays combine historical, doctrinal, critical and sociological forms of legal analysis to provide important insights into an issue that continues to challenge international legal theory and practice.