What does the Fourth Amendment protect against?

The Fourth Amendment reads: “The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.” For most of its history, courts read this text through a property lens: a Fourth Amendment “search” required physical trespass onto someone's protected space. Katz v. United States (1967) changed that framework. FBI agents had attached a listening device to the outside of a public phone booth Charles Katz used to place illegal betting calls — no trespass onto anything Katz owned. The Supreme Court held that didn't matter: “the Fourth Amendment protects people, not places,” and what a person “seeks to preserve as private, even in an area accessible to the public, may be constitutionally protected.” Justice Harlan's concurrence supplied the test still used today: a search occurs when the government intrudes on a place or communication where a person has both a subjective expectation of privacy and one society recognizes as reasonable. That reframing is why the Fourth Amendment now reaches wiretaps, thermal imaging of a home, and cell-site location data, none of which existed or were anticipated in 1791 — the protection travels with the person's reasonable expectation of privacy rather than staying tied to physical boundaries. It's also why the amendment isn't absolute: searches with a valid warrant, or that fall under recognized exceptions like consent or exigent circumstances, remain constitutional.