In every American jurisdiction, new rules of law announced by a court are presumed to have retrospective effect—that is, they are presumed to apply to events occurring before the date of judgment. There are, however, exceptions in certain cases where a court believes that such application of the new rule will upset serious and reasonable reliance on the prior state of the law. This essay, a substantially abridged version of the United States Report on the subject, submitted at the Nineteenth International Congress of Comparative Law, summarizes these exceptional cases. It shows that the proper occasions for issuing exclusively or partially prospective judgments have varied over time and that there are still substantial differences in approach according to the particular jurisdiction and the kind of law under consideration. It concludes with a brief survey of some of the still unresolved jurisprudential and constitutional problems raised by recognition