This completely revised and rewritten handbook gives an overview of international organization (IO) as a dynamic field of research that adds to our understanding of global and regional relations and related domestic politics. Bringing together international scholars from a range of disciplines, it considers both IO as a process and multilateral organizations as institutions. This handbook is divided into five parts: I. Documentation, sources and perspectives II. International secretariats as bureaucracies III. Actors within and beyond international bureaucracies IV. Processes within and beyond international bureaucracies V. Challenges to international organizations Containing new chapters on topics such as the anthropological perspective, IO secretariats in several continents outside of Europe, feminization, the digital turn and challenges to IO legitimacy, the contributors reflect on the progression of IO studies from a burgeoning field to a well‐established subfield of international relations and the move away from scholarship based mainly in North‐Western Europe and the United States. This book will be of particular interest to scholars and students of IOs, global governance, diplomacy and foreign policy, as well as practitioners of multilateral cooperation.