This pioneering Research Handbook with contributions from renowned experts, provides an overview of the general doctrines making up the law of international organizations. The approach of this book is taken from a novel perspective: that of the tension between functionalism and constitutionalism. In doing so, this Handbook presents not only practically relevant information, but also provides a tool for understanding the ways in which international organizations work. It has separate chapters on specific 'constitutional' topics and on two specific organizations: the EU and the UN. Research Handbook on the Law of International Organizations will be of particular interest to academics and graduate students in the fields of international law, international politics and international relations.