This book deals with the nature of international organisations and the tension between their legal nature and the system of classic, state-based international law. This tension is important in theory and practice, particularly when organisations are brought under the rule of international law and have to be conceptualised as legal subjects, for example in the context of accountability. The position of organisations is complicated by what the author terms 'the institutional veil', comparable to the corporate veil found in corporate law. The book focuses on the law of treaties, as this pre-eminently 'horizontal' branch of international law brings out the problem particularly clearly. The first part of the book addresses the legal phenomenon of international organisations, their legal features as independent concepts, the history of international organisations and of legal thought in respect of them, and the development of contemporary law on international organisations. The second part deals with the practice of international organisations and treaty-making. It discusses treaty-making practice within organisations, judicial practice in interpretation of organisations' constitutive treaties, and the practice of treaty-making by organisations. The third and final part analyses the process by which international organisations have been brought under the rule of the written law of treaties, offering a practical application of the conceptual framework as previously set out. Part three is at the same time an analytic overview of the drafting history of the 1986 Vienna Convention on the Law of Treaties between States and International Organizations or between International Organizations. This is a profound and penetrating examination of the character of international organisations and their place in international law, and will be an important source for anyone interested in the future role of organisations in the international legal system.