International organisations are a central component of modern international society. This book provides a concise account of the principles and norms of international law applicable to the intergovernmental organisation (IGO). It defines and explains inter-governmentalism and the role of law in its regulation. The book presents case studies that show how the law works within an institutional order dominated by politics. After a note on the key relationship between the IGO and its member states, it examines the basic relationship between the UN and states in terms of membership through admissions, withdrawal, expulsion, suspension, and representation. The debate about the extent of the doctrine of legal powers is addressed through case studies. Institutional lawmaking in the modern era is discussed with particular focus on at the impact of General Assembly Resolutions on outer space and the Health Regulations of the World Health Organization. Non-forcible measures adopted by the UN and similar IGOs in terms of their legality (constitutionality and conformity to international law), legitimacy and effectiveness, is covered next. The different military responses undertaken by IGOs, ranging from observation and peacekeeping, to peace enforcement and war-fighting, are discussed in terms of legality and practice. The book also considers the idea of a Responsibility to Protect and the development of secondary rules of international law to cover the wrongful acts and omissions of IGOs. It ends with a note on how the primary and secondary rules of international law are upheld in different forms and mechanisms of accountability, including courts.