This book offers the first comprehensive study of unjust enrichment as a principle of international law.While domestic legal systems have long recognised unjust enrichment as a basis for restitution, its role in international law remains underexplored. This book addresses this gap by examining the principle’s status as a general norm of international law and its implications for inter-state relations, investment arbitration, and the interpretation of other international rules. The book advances three key arguments: that unjust enrichment claims in international law may extend to indirect recipients; that states enriched through conduct harming another state can incur restitutionary obligations even without a direct transfer; and that the principle of unjust enrichment is not merely subsidiary to the law of state responsibility. Through rigorous analysis, the book explores the rationales underlying unjust enrichment – proprietary, fairness, and wrongfulness – and evaluates their applicability in the international arena. It also considers practical challenges such as indeterminacy, multiple-claimant situations, and the risk of proliferating disputes. Drawing on jurisprudence from the Iran-US Claims Tribunal, ICSID, and other international tribunals, as well as comparative insights from English, German, USA, and other domestic laws, the book provides a nuanced framework for applying unjust enrichment in international law. It will be indispensable for scholars of public international law, practitioners engaged in inter-state adjudication, government lawyers, and investment arbitration specialists seeking principled solutions to complex disputes.