The importance of the law of international organisations is continually rising. The textbook which was first edited in German in 2009 explains and analyses not only the structures of international organisations in general, but it focuses upon the interplay between the creation of institutional structures and important substantive areas of public international law. Thus, in the first and second part of the book the general aspects of the law of international organisations are presented, whereas in a third part international security, human rights protection, economy, development and environmental protection are analysed as regards the interplay between substantive and institutional aspects. This part is built on the assumption that the law of international organisations needs to be studied "in action", i.e. by looking at highly institutionalised areas of international law in order to analyse the mutual influences between institutional and substantive international law. In fact, today all important parts of international law are institutionalised in different international organisations, a phenomenon which is reflected in the title. Up to now, there is no other book on international institutional law which brings together institutional and substantive aspects in a comparable manner. The textbook is directed to students of the law of international organisations but also to students in the social sciences, above all, political science. It shall also be useful to practitioners in the field of international institutions.