AbstractThis article examines the principle of non-retroactive application of law, which prohibits the application of law to events that took place before the law was introduced. The application of this principle has become particularly controversial as states adopt stricter regulations to tackle climate change with retroactive effect, and investors challenge such regulations before international courts and tribunals. In the context of criminal law, the principle is widespread and has become a binding norm of international law. However, a survey of domestic jurisdictions and decisions of international courts and tribunals shows that that there is no general principle of international law which forbids the retroactive application of administrative law. Despite pronouncements of some international courts and tribunals to the contrary, states can conclude treaties and adopt administrative regulations with retroactive effect to pursue legitimate public policy objectives.