This book offers an analysis of the law of treaties as it emerges from the interplay between the 1969 Vienna Convention on the Law of Treaties and customary international law. It revisits the basic concepts underlying the provisions of the Vienna Convention, so as to determine the actual state of the law and its foreseeable development. In doing so, it examines some of the most controversial aspects of the law of treaties. The book first explores the influence exerted by the Vienna Convention on pre-existing customary law. Certain rules of the Convention which, at the time of its adoption, appeared to fall within the realm of progressive development, can now be regarded as customary international rules. Conversely, a number of provisions of the Convention, in particular those which have been the subject of subsequent codification work by the International Law Commission, have become obsolete. It then examines the impact exerted by the Vienna Convention on the development of other fields of international law, such as the law of international responsibility and the law of international organizations. The last section of the book is devoted to cross-cutting issues, with particular reference to the notion of jus cogens — a concept first used in the Vienna Convention in connection with the problem of the validity of treaties and which, afterwards, has acquired a legal significance going well beyond the Convention.