This book provides a concise account of the principles and norms of international law applicable to the main-type of international organisation - the inter-governmental organisation (IGO). That law consists of principles and rules found in the founding documents of IGOs along with applicable principles and rules of international law. The book also identifies and analyses the law produced by IGOs, applied by them and, occasionally, enforced by them. There is a concentration upon the United Nations, as the paradigmatic IGO, not only upon the UN organisation headquartered in New York, but on other IGOs in the UN system (the specialised agencies such as the World Health Organisation).