Federal agents installed a GPS device on Antoine Jones’s Jeep one day after the warrant’s deadline and outside the warrant’s authorized District of Columbia location, then monitored the vehicle’s movements for 28 days. The resulting data helped connect Jones to a drug-trafficking conspiracy and contributed to his conviction and life sentence. The District Court suppressed only data obtained while the Jeep was at Jones’s residence, reasoning that travel on public roads carried no reasonable expectation of privacy; the D.C. Circuit reversed. The Supreme Court affirmed. It held that the government’s physical occupation of the Jeep—an effect protected by the Fourth Amendment—for the purpose of obtaining information was a search under the Amendment’s common-law trespassory understanding. The Court explained that Katz’s reasonable-expectation-of-privacy test supplemented rather than displaced that property-based protection, and it declined to decide the government’s alternative reasonableness argument because the government had forfeited it below. Justice Sotomayor emphasized the privacy risks of inexpensive, comprehensive GPS surveillance, while Justice Alito would have treated the four-week monitoring itself as a search under Katz. The Court affirmed the judgment below.