This book deals with a central issue of international law: the relationship between two of its sources, treaty and custom. In particular, it examines one aspect of this relationship that has not been satisfactorily covered in the literature, whether new customary law may abrogate or modify prior incompatible treaties. State practice in the Law of Sea and other areas of international law contains a number of examples of treaties that have been terminated or revised on account of new conflicting custom. The author draws on these examples as well as on decisions of international tribunals to argue that although new customary law does not automatically affect prior incompatible treaties, it gives a State, under certain conditions, the unilateral right to call for their termination or revision. This is an original position on a controversial subject that was considered too complex to be included in the Vienna codification of the Law of Treaties. This issue may arise in all areas of international law and is of practical importance to all those involved in the field.