Leading up to the Revolution, a series of abuses by King George III and his representatives led to the widespread recognition in the colonies of a right against unreasonable searches and seizures by the government. Several state constitutions adopted such protections soon after the Declaration of Independence was proclaimed. In 1780, for example, the Constitution of Massachusetts announced that every individual has “a right to be secure from all unreasonable searches and seizures of his person, his house, his papers, and all his possessions.” The Fourth Amendment text ratified in 1791 closely resembled these state provisions in its recognition of the “right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures.”

The most significant abuse that had led to the adoption of the Fourth Amendment was the execution of general warrants in the colonies. General warrants were court orders authorizing government officials to search and seize evidence with few if any limitations on where the officials could search and what they could seize. Opposition to general warrants had derived in part from two famous English cases, Entick v. Carrington (1765) and Wilkes v. Wood (1763). Both cases involved pamphleteers who were critics of the government. They were arrested and their books and papers were seized (including, in John Wilkes’ case, all the papers of forty-nine of his friends) using warrants that named neither the suspects nor the places to be searched. Both defendants sued the seizing agents for trespass and won judgments in their favor. The latter half of the text of the Fourth Amendment is directly addressed to abolishing the general warrant used in cases like Entick and Wilkes. Its plain language requires warrants to be narrow, as “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.” The Fourth Amendment was also inspired by colonial opposition to the writs of assistance, which permitted the customs agents to search any place in which smuggled goods might be concealed, even if there was no particular suspicion the goods were there. In a famous case, known as Paxton’s Case (1761), widely known among the Framers of the Constitution, James Otis defended several colonial smugglers against seizures made through the use of these writs. Although Otis lost the case, no less an authority than John Adams— who watched Otis in court—saw the dispute as the spark of the American Revolution.

Despite the general agreement among Supreme Court decisions and legal historians that one major purpose of the Fourth Amendment was to abolish general warrants and writs of assistance, there is widespread disagreement about the original understanding of the specific terms and purposes of the Fourth Amendment. For example, Thomas Davies argues that the Fourth Amendment was largely focused on abolishing general warrants, and that it is wrong to see the text of the Fourth Amendment as imposing a more general requirement of reasonable police practices. William Cuddihy argues that the Fourth Amendment was originally understood both to prohibit general warrants and more generally to guard against abusive law enforcement practices. On the other hand, Akhil Amar argues that the original understanding of the Fourth Amendment was to require reasonableness in government investigations.

The Fourth Amendment lay largely dormant until the twentieth century. That was true for three major reasons. First, the Fourth Amendment applied only to the federal government, which at the time was both relatively small and had few resources devoted to law enforcement. Second, the concept of modern police officers hired by the government to investigate crime did not emerge until the middle of the nineteenth century. Third, the remedies for violations of the Fourth Amendment remained uncertain until the introduction of the exclusionary rule in Weeks v. United States (1914) (evidence unlawfully seized may not be introduced into evidence). Before the prohibition era in the 1920s, the lack of a clear remedy, the absence of modern police forces, and the narrow role of the federal government ensured that Fourth Amendment issues arose only rarely.

The opposite is true today. Fourth Amendment questions now arise in an extraordinary number of cases, and they are litigated with unusual frequency. The inversion of the three reasons above explains why. First, since Wolf v. Colorado (1949) and Mapp v. Ohio (1961), the Supreme Court applied the Fourth Amendment (and later the exclusionary rule) to state and local police officers under the theory that they were “incorporated” by the Fourteenth Amendment. Second, federal and state law enforcement has become a remarkably vast enterprise: there are more than 750,000 state and local officers with the power to make arrests, and upwards of 100,000 federal officers with that power. Third, Fourth Amendment remedies now include both a modified exclusionary rule and civil remedies in federal court. The result is an extremely large body of modern Fourth Amendment case law.

The threshold question under the Fourth Amendment is whether a government search or seizure has occurred. A person’s property is “seized” when the government meaningfully interferes with a person’s possessory interest in their property. United States v. Jacobsen (1984). This occurs when the government takes a suspect’s property away; when the government forces a person out of their home; or when the government takes a package or letter out of the course of delivery. Similarly, a person is “seized” under the Fourth Amendment when the government terminates or restrains his freedom of movement through means intentionally applied. Brendlin v. California (2007). This happens when a government official places a suspect under arrest or temporarily detains a person in circumstances where a reasonable person would not feel free to terminate the encounter and leave.

The question of when government conduct is a Fourth Amendment “search” has received a enormous amount of judicial attention. The doctrine recognizes two tests. First, government conduct is a search if it is a trespass onto a person, his house, his papers, or his effects with the intent to obtain information. United States v. Jones (2012). Second, under the test first announced in a concurring opinion in Katz v. United States (1967), government conduct is a search if it violates a subjective expectation of privacy, and society is prepared to recognize that expectation as objectively reasonable. For the most part, government conduct usually ends up being labeled a search if it is an invasion into a private space such as a home, a car, a package, a letter, or a person’s pockets. On the other hand, government conduct is not labeled a search if it involves surveillance in public.

Once courts recognize a search or seizure, the next question is whether the search or seizure is constitutionally “reasonable” or “unreasonable.” Likewise, a person can be searched without a warrant incident to his arrest, but absent exigent circumstances a warrant is required to search the digital contents of an arrestee’s cell phone. United States v. Robinson (1973); Riley v. California (2014). In the case of persons, brief seizures are reasonable with specific and articulable facts, while arrests are reasonable based on probable cause to believe a crime has been committed and that person has committed it. United States v. Watson (1976); Terry v. Ohio (1968). After a person has been arrested, he can be searched fully without a warrant incident to arrest. United States v. Robinson (1973).

The rules for searching homes tend to be somewhat more restrictive than the rules for seizing and searching individuals. In the case of homes, entrance is constitutionally reasonable (and therefore legal) only pursuant to a valid warrant or an exception to the warrant requirement. The exceptions to the warrant requirement include exigent circumstances or consent by a party who has authority over the space to be searched. If the police are lawfully in a position to view evidence outside the scope of a warrant but its incriminating nature is immediately apparent, the police can seize that evidence under the “plain view” exception. Horton v. California (1990).

The rules for searching and seizing cars tend to give the government significantly broader powers than the rules for homes. A police officer can order a driver to pull over a car, seizing the car and its occupants, based on any traffic violation. Whren v. United States (1996). The officer can then arrest the driver based on only a minor violation, even if the crime of arrest does not provide for any jail time. Atwater v. City of Lago Vista (2001). Once the person has been arrested, he can be searched incident to arrest even if the arrest violates state law. Virginia v. Moore (2008). Further, the car can be searched without a warrant if there is probable cause to believe that contraband or evidence is located inside it. Carroll v. United States (1925).

After a Fourth Amendment violation has been established, the remaining question is whether there is a remedy for the violation in a court of law. The exclusionary rule remains one remedy, although it is subject to many exceptions and those exceptions appear to be expanding over time. In general, a defendant can successfully invoke the exclusionary rule only if his own rights were violated and the constitutional violation was the direct cause of the evidence being discovered. Also, the Supreme Court has recently developed the so-called “good faith” exception to the exclusionary rule. Under the most recent precedents, the exclusionary rule is a last resort available only when the officer who conducted the search acted in a personally culpable manner, such as in the case of a knowing or intentional violation. Davis v. United States (2011). Civil suits against officers who conducted unconstitutional searches or seizures have similar limits. Under the doctrine of qualified immunity, an officer is immune from suit unless the violation was clearly established at the time and a reasonable officer would have recognized that the act violated the Constitution. Anderson v. Creighton (1987).