The fourth edition of this market-leading textbook offers students a clear framework for understanding the practice and logic of International Organizations Law. It is structured around the three defining relationships IOs engage in - namely, with their member states, with their organs and staff, and with the outside world. These different dynamics give rise to different concerns, which each help to explain the logic behind international institutional law. The text also discusses the essential topics of the law of IOs, including powers and finances, privileges and immunities, institutional structures, and accountability. By demonstrating how the theory works in practice, with recent examples, students will observe the impact and significance of International Organizations Law. Updated with the latest case law and literature, this new edition also contains discussions of the withdrawal of Israel and the US from UNESCO, Brexit, and the Covid-19 pandemic, and how these affect the law of international organizations.