In Republic of Austria v. Altmann, the U.S. Supreme Court decided that the Foreign Sovereign Immunities Act of 1976 (FSIA) generally applies to claims based on events that occurred before the Statute\u27s enactment. To decide the retroactivity question, the Court had occasion to consider the essential nature of foreign sovereign immunity: is it merely a procedural immunity providing foreign states with present protection from the inconvenience and indignity of a lawsuit, or is it something more than that? The Court\u27s examination of this question was brief and unsatisfying. Its analysis would have been enriched by a recognition that foreign sovereign immunity is regulated not just by federal statute, but also by principles of customary international law that the federal statute sought, in large part, to codify. Among the authorities the Court did consider, it found support for the proposition that foreign sovereign immunity is a procedural immunity and also for the proposition that foreign sovereign immunity is an immunity from substantive liability. Viewing these authorities as contradictory, the Court concluded that the retroactivity issue had to be resolved on other grounds. This brief article maintains that the relevant authorities are not contradictory. They are consistent with the conclusion that foreign states enjoy both a procedural and a substantive immunity, a possibility that the Court appears to have overlooked