The U.S. Supreme Court’s decision in Carpenter v. United States is the latest in a line of cases in which the Court has applied the Fourth Amendment in light of changed technological circumstances. Commentators have variously described the decision as groundbreaking or incremental. In fact, the decision has the potential to be either. By holding that individuals have a reasonable expectation of privacy in historical cell-site locational information (‘‘CSLI’’) held by cellular service providers, the Court limited the so-called third-party doctrine and suggested that aspects of new digital technologies alter the constitutional privacy analysis in ways not yet clearly reﬂected in statutes or cases. At the same time, the Court explicitly stated that its opinion is ‘‘narrow,’’ thereby tempering expectations of a sea change in Fourth Amendment jurisprudence. Despite this attempt to downplay the opinion’s signiﬁcance, however, the Court’s tightening of the third-party doctrine, which generally denies Fourth Amendment protection for information provided to third parties, may well make Carpenter the most important privacy decision in a generation. At the very least, the opinion is sure to spawn debate in lower courts about how far its rationale should extend. In the meantime, holders of digital data should seek to understand the decision’s impact on their businesses and monitor how lower courts and government ofﬁcials apply its pronouncements in practice.