Conventional wisdom suggests that a constitutional right should be defined so as to effectively constrain government actors. A right defined in terms of what state actors routinely do would seem to impose in practice an ineffectual brake on much intrusive state action—and so seems pointless. Nevertheless, in defining Fourth Amendment rights, the Supreme Court frequently draws on the practice of contemporaneous government actors to define the constitutional floor for police action. The actions of the regulated thus define the content of regulation. This Article isolates and analyzes this seemingly paradoxical judicial practice, which it labels “Fourth Amendment gloss,” by analogy to methodological practices elsewhere in constitutional law. The latter is examined through a comparison to a similar, albeit not identical, mode of reasoning used in separation of powers cases. The justifications for gloss in the latter domain are more fully developed and hence provide useful benchmarks for evaluation of Fourth Amendment gloss. The Article’s first aim is descriptive—to catalog the various ways in which “gloss,” or official practice, is deployed across the Court’s search and seizure case law. This exercise shows that many frequently exercised search and seizure powers have been constitutionally defined in terms of official practice. The Article’s second aim is to ask whether judicial reliance on such gloss can be justified. There are three general justifications for the use of official practice as a source of law in constitutional interpretation. These can be labeled the acquiescence, Burkean, and settlement justifications. A careful examination of the empirical and theoretical contexts of the Fourth Amendment suggests, however, that none of these three justifications supports gloss’s use as a way to define lawful searches and seizures. If gloss persists today, therefore, it is for institutional and ideological reasons—not because it is theoretically warranted. Given this conclusion, the Article offers ways to limit the error costs associated with the use of Fourth Amendment gloss.