When does a State’s promise become legally binding? Governments make promises in speeches, diplomatic correspondence, and legislation – but not all create obligations under international law. Distinguishing binding promises from policy statements and political rhetoric is notoriously difficult. This book shows you how to tell the difference. Drawing on case law, State practice, and scholarship, it asks which criteria determine whether an oral or written promise binds a State vis‑à‑vis other States or foreign investors, who has the authority to make such commitments, and when States can walk away from them. It offers a comparative analysis of key concepts in general international law and investment law to identify the principles governing promises across both fields.