This volume is the up-to-date English version of the fifth Italian edition of a textbook on the United Nations which was first published in 1971 by CEDAM (Padua). The book aims to provide a comprehensive legal analysis of problems concerning membership, the structure of U.N. organisations, their functions and their acts taking into consideration the text of the Charter, its historical origins, and, particularly, the practice of the organisations. Developments in United Nations practice subsequent to 1971 have obviously been taken into account. As a general working criterion, the more recent practice has been added to the pre-existing one, rather than substituting it, even when past practice may appear to be obsolete. Indeed one of the aims of the book is to trace the `story' of the United Nations from its birth precisely through an analysis of the practice. Moreover, since the Charter has never undergone any substantive modifications, one cannot exclude that what may appear to be old and obsolete today could become of current interest in the future. For this reason the examination of former practice will sooner or later become useful to anyone seeking to interpret the Charter. For instance, in 1975 the United States proposed the admission of the two Vietnams, which were separate countries at that time, and of the two Koreas, under the `package' technique. Thus the well-known 1948 advisory opinion of the International Court of Justice on the `package' proposed by the Soviet Union for collective admission in the 40s and 50s again became timely, despite its having appeared obsolete. This legal analysis which is free of dogmatism and firmly linked to practice describes the role played by the United Nations in the past and at present better than many lengthy and inconclusive political or sociological studies. The book is very much focused on the Charter as it stands while it only marginally deals with reforms that might be introduced, such as those concerning the structure of the Security Council and the General Assembly. Indeed, it is difficult to foresee radical reforms, giving the UN an entirely new shape. This is particularly true with regard to endowing the Organisation with the force and efficacy that would be needed for the maintenance of peace and security. Recent events have clearly shown how unfeasible such an endowment would be.