Abstract Taking an approach to international law that embraced key features of the New Haven School, Rosalyn Higgins has made several important contributions to the law of international organizations. In 1963, at a time when no theory had addressed the work of the United Nations’ (UN) political organs, Higgins showed how General Assembly resolutions and other decisions contribute to the development of customary international law. She also demonstrated that, separate and apart from UN member states, the UN Secretariat was contributing to the development of customary international law regarding treaties. Later, as counsel for the International Tin Council and later rapporteur for the Institut de Droit International, Higgins put on the global agenda the issue of the accountability of international organizations for harm to third parties, highlighting gaps in existing law and competing policy concerns that precluded easy solutions. Higgins made two contributions in marking the path forward for theorizing about international organizations, one substantive and the other methodological. She highlighted the importance of considering international organizations in relation to the full range of actors with whom they interact, including private parties. As a methodological matter, Higgins demonstrated the value of an inductive approach where legal claims are grounded in close, detailed and careful observations of how international organizations actually operate.