Cooperation through international organizations is fundamental to the international legal order. International organizations are nowadays ubiquitous and come in many different manifestations, each allowing for different levels of international cooperation. The profile of regional and universal organizations may vary greatly from one organization to another. At the same time, they do not live apart and this has led to the creation of a complex network of relationships. These relationships have seldom been the object of scholarship, and this book seeks to address that gap. In general, the relationships between international organizations can give rise to such issues as the conditions placed upon one organization by another, demarcations of competence, membership of other organizations, and various forms of collaboration involving the conclusion of agreements between organizations. Optimal coexistence, cooperation and coherence all play a role in optimizing the relations between international organizations. The volume concludes by analysing current challenges, including those of legal identity, responsibility and accountability, as well as making proposals for reform, such as through the development of a common law between organizations.