This textbook, first published in German, explains and analyses not only the structures of international organisations in general, but also focuses on the interplay between the creation of institutional structures and important substantive areas of public international law. In the first and second parts of the book the general aspects of the law of international organisations are surveyed, and in the third part international security, human rights protection, trade, development and environmental protection are analysed in terms of the interplay between substantive and institutional law. This third part is built on the assumption that the law of international organisations needs to be studied 'in action', ie by looking at highly institutionalised areas of international law as a way of analysing the mutual influences between institutional and substantive international law. This is the first book on international law to bring together institutional and substantive aspects in this comparable manner. It is aimed at students of the law of international organisations, the social sciences and political science and practitioners in the field of international institutions.