What is the Fourth Amendment?

The Fourth Amendment reads in full: '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.' It was ratified on December 15, 1791, as one of the first ten amendments -- the Bill of Rights. The amendment has two clauses: the Reasonableness Clause ('shall not be violated') and the Warrant Clause ('no Warrants shall issue, but upon probable cause'). Courts have long debated how the two clauses relate to each other -- whether all searches must meet warrant requirements to be reasonable, or whether the Reasonableness Clause sets an independent standard that may sometimes be satisfied without a warrant. In practice, the Supreme Court has recognized a large and expanding body of warrant exceptions: searches incident to lawful arrest, the automobile exception, exigent circumstances, consent, and the plain-view doctrine, among others.

The exclusionary rule -- the doctrine that evidence obtained in violation of the Fourth Amendment cannot be used against the defendant in a criminal trial -- was established as a matter of federal constitutional law in Weeks v. United States, 232 U.S. 383 (1914). For four decades after Weeks, the exclusionary rule bound only federal agents; state courts could admit illegally seized evidence under the so-called 'silver platter' doctrine. The Supreme Court changed course in Mapp v. Ohio, 367 U.S. 643 (1961). Police in Cleveland, Ohio, had forced their way into Dollree Mapp's home without a valid warrant and discovered obscene materials. Justice Tom Clark, writing for the Court, held that the exclusionary rule is an essential part of the Fourth Amendment's protection and applies equally to state and local law enforcement through the Due Process Clause of the 14th Amendment -- the same incorporation mechanism used for other Bill of Rights guarantees. Mapp made the exclusionary rule a nationwide rule binding on all government actors. The Court subsequently adopted a 'good faith' exception in United States v. Leon, 468 U.S. 897 (1984), holding that evidence need not be suppressed if the officers conducting the search reasonably relied in good faith on a warrant that was later found to be invalid.

The modern framework for defining what the Fourth Amendment protects was established in Katz v. United States, 389 U.S. 347 (1967). Federal agents had attached an eavesdropping device to the outside of a public telephone booth to record the defendant's calls without obtaining a warrant, relying on the theory that the Fourth Amendment protects places, not people, and that no physical trespass into the booth had occurred. The Court rejected that theory, holding that the Fourth Amendment 'protects people, not places' and that what a person knowingly exposes to the public is not protected, but what they seek to preserve as private, even in a public area, may be protected. Justice John Marshall Harlan II's concurrence articulated the influential two-part test that has defined Fourth Amendment analysis ever since: a search occurs when the government invades a subjective expectation of privacy that society is prepared to recognize as reasonable. Under this 'reasonable expectation of privacy' framework, the constitutional question is not whether a physical intrusion occurred but whether the person had a legitimate privacy interest in the information or place searched.

Terry v. Ohio, 392 U.S. 1 (1968), addressed what happens when police have reason to suspect criminal activity but lack the probable cause required for a full arrest or a traditional search warrant. Chief Justice Earl Warren, writing for the Court, held that a police officer may briefly detain a person for investigatory questioning -- a 'Terry stop' -- based on specific and articulable facts that criminal activity may be afoot, a standard called 'reasonable suspicion' that is less demanding than probable cause. The officer may also conduct a limited pat-down of the suspect's outer clothing for weapons -- a 'frisk' -- if the officer reasonably believes the person is armed and dangerous. Reasonable suspicion falls between a hunch (not enough) and probable cause (not required for a brief stop). Terry stop-and-frisk practices have been a recurring subject of policy debate, particularly in cities that adopted aggressive street-stop programs, and the scope of reasonable suspicion in practice has been disputed in civil rights litigation and political campaigns alike.

The Supreme Court has confronted a series of cases asking how the Fourth Amendment applies to digital technology that was not contemplated when the amendment was ratified in 1791. In Riley v. California, 573 U.S. 373 (2014), Chief Justice John Roberts wrote a unanimous opinion holding that police must obtain a warrant before searching the contents of a cell phone seized incident to a lawful arrest. The Court rejected the government's argument that cell phones could be searched like any other item found on an arrested person, noting that modern cell phones contain vast quantities of sensitive personal information -- call logs, photos, emails, health data, financial records, location history -- that bears no comparison to the contents of a wallet or briefcase. In Carpenter v. United States, 585 U.S. 296 (2018), Chief Justice Roberts again wrote the majority (5-4) holding that the government must generally obtain a warrant supported by probable cause before compelling a wireless carrier to produce historical cell-site location information (CSLI) -- records that can reconstruct a person's detailed physical movements over days or months. The Court declined to extend the third-party doctrine -- the principle that information voluntarily shared with third parties loses Fourth Amendment protection -- to the comprehensive location tracking that CSLI enables. Riley and Carpenter together signal that the Court is prepared to apply the Fourth Amendment's warrant requirement to categories of digital data that carry the functional equivalent of home or diary surveillance, even when that data is technically held by a third-party service provider.

For the 2028 presidential election, the Fourth Amendment is relevant across several active policy debates. First, government surveillance: the programs that collect communications metadata in bulk, the legal frameworks under the Foreign Intelligence Surveillance Act (FISA), and executive-branch interpretation of the Fourth Amendment's national-security exception have been debated by both parties since at least the early 2000s. A 2028 president will oversee executive branch agencies whose surveillance programs are constrained -- or not -- by the Fourth Amendment and its statutory analogs. Second, stop-and-frisk and policing policy: local law enforcement tactics that involve investigatory stops remain contested, and presidential candidates typically take positions on federal law enforcement priorities, police reform, and qualified immunity for officers. Third, digital privacy: the pace of technological change -- from artificial intelligence-based surveillance to biometric data collection to warrantless access to third-party app data -- continues to raise new Fourth Amendment questions that the courts are resolving in real time. Fourth, the exclusionary rule itself: proposals to narrow or abolish the exclusionary rule in favor of alternative remedies for constitutional violations appear periodically in legal scholarship and policy debate. The 2028 winner will appoint federal judges who will resolve these questions in the years ahead.