Greater, lesser, or just different than the sum of their parts? For all their prominence in global affairs, international organizations remain relative strangers from the perspective of international legal theory. Drawing insights from philosophical discourse, this book moves past binary models that would have international organizations either be nothing over and above their members or simply analogous to them. Rather than compare international organizations and their members, Chasapis Tassinis asks us to understand them both as manifestations of communal organization and what international law recognizes as 'public' authority. Theorizing international organizations as only a branch within a broader family of corporate entities, this book allows us to untangle old doctrinal puzzles. These include the extent to which international organizations are bound by customary international law and can contribute to its formation, or whether they enjoy a legal personality that is opposable to members and non-members alike.