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 offers a framework for making that distinction. 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.