This volume contains the first comprehensive survey of practice relating to cases of State succession and membership in international organizations between 1945 and 2000. The study closes a lacuna of legal research in a field of State succession that has been left aside by the International Law Commission in its codificatory work. The practice examined by the author includes most controversial and topical cases, ranging from i.a. the acquisition of independence of the Philippines and British India, through the unification of the United Arab Republic, Vietnam, Yemen, and Germany, to the dissolution of the former Soviet Union and Yugoslavia. In the concluding chapter basic principles of State succession and membership as well as criteria for a classification of international organizations are identified. After a confrontation of the confusing practice, which is mostly guided by political pragmatism, with the traditional legal theories, the author proposes a new functional approach to State identity with respect to membership in international organizations.