The book investigates how an analogy between States and international organizations has influenced and supported the development of the law that applies to intergovernmental institutions on the international plane. That is best illustrated by the work of the International Law Commission on the treaties and responsibility of international organizations, where the Commission for the most part extended to organizations rules that had been originally devised for States. Revisiting those codification projects while also looking into other areas, the book reflects on how techniques of legal reasoning can be - and have been - used by international institutions and the legal profession to tackle situations of uncertainty, and discusses the elusive position that international organizations occupy in the international legal system. By cutting across some foundational topics of the discipline, the book makes a substantive contribution to the literature on subjects and sources of international law.