The United Nations (UN) has expanded beyond all recognition since its founding in 1945. This volume represents a study that is entirely new, but is prepared in the way that has become so familiar over succeeding editions of Oppenheim’s International Law. It covers the formal structures of the UN as it has expanded over the years, and all that this complex organization does. All substantive issues are addressed in separate sections, including the responsibilities of the UN, financing, immunities, human rights, preventing armed conflicts, peacekeeping, and judicial matters. In examining the evolving structures and ever-expanding work of the UN, this volume follows the long-held tradition of Oppenheim by presenting facts uncoloured by personal opinion, in a succinct text that also offers in the footnotes extra information and ideas to be explored. It is a book that, while making all necessary reference to the UN Charter, the Statute of the International Court of Justice, and other legal instruments, tells of the realities of the legal issues as they arise in the day-to-day practice of the UN.