This chapter examines the legal framework governing international organizations. It begins with an examination of the history, role, and nature of international organizations. It is argued that although the constituent instruments and practices of each organization differ, there are common legal principles which apply to international organizations. The chapter focuses on the identification and exploration of those common legal principles. There is an examination of the manner in which international organizations acquire legal personality in international and domestic law and the consequences of that legal personality. There is also discussion of the manner in which treaties establishing international organizations are interpreted and how this differs from ordinary treaty interpretation. The legal and decision-making competences of international organizations are considered as are the responsibility of international organizations and their privileges and immunities. Finally, the chapter examines the structure and powers of what is the leading international organization—the United Nations (UN).