Unjust Enrichment in International Law

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 roles of the international norm against unjust enrichment both as an independent basis for a claim and in the development and interpretation of other international norms. It explores the far-reaching implications of this norm for inter-state relations and investment arbitration.

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 liability in unjust enrichment even without a transfer of value from the harmed state to the enriched state; and that the principle of unjust enrichment is not merely subsidiary to the law of state responsibility for internationally wrongful conduct. 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, US, 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.