Carpenter v. United States, the 2018 Supreme Court opinion that requires the police to obtain a warrant to access an individual’s historical whereabouts from the records of a cell phone provider, is the most important Fourth Amendment opinion in decades. Although many have acknowledged some of the ways the opinion has changed the doctrine of Constitutional privacy, the importance of Carpenter has not yet been fully appreciated. Carpenter works many revolutions in the law, not only through its holding and new rule, but in more fundamental respects. The opinion reinvents the reasonable expectation of privacy test as it applies to large databases of information about individuals. It turns the third-party doctrine inside out, requiring judges to scrutinize the products of purely private decisions. In dicta, it announces a new rule of technological equivalence, which might end up covering more police activity than the core rule. Finally, it embraces technological exceptionalism as a centerpiece for the interpretation of the Fourth Amendment, rejecting backwards-looking interdisciplinary methods such as legal history or surveys of popular attitudes. Considering all of these revolutions, Carpenter is the most important Fourth Amendment decision since Katz v. United States, a case it might end up rivaling in influence.