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 inter-national 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 monograph reflects on how tech-niques of legal reasoning can be – and have been – used by international institutions and the le-gal profession to tackle situations of uncertainty, and discusses the elusive position that interna-tional organizations occupy in the international legal system. It cuts across some foundational topics of the discipline, making a substantive contribution to the literature on subjects and sources of international law.