The sources discuss the Fourth Amendment of the U.S. Constitution, which safeguards individuals against unreasonable searches and seizures by the government. They explain that a search occurs when a reasonable expectation of privacy is violated, and a seizure involves meaningful interference with a person's possessory interests or freedom of movement. While a warrant based on probable cause is generally required, numerous exceptions to the warrant rule exist, such as exigent circumstances, consent, and searches incident to arrest. The exclusionary rule serves as the primary remedy for violations, rendering illegally obtained evidence inadmissible in court, though it too has several exceptions. The sources also highlight the evolving interpretation of the Fourth Amendment in light of advancing technology.

Olmstead v. United States narrowly defined a search based on physical trespass, meaning wiretapping without physical intrusion was not a search. Katz v. United States broadened this, holding that a search occurs when the government violates a person's "reasonable expectation of privacy," shifting the focus from places to people.

Justice Harlan's two-prong test states that a search occurs if (1) the individual exhibited a subjective expectation of privacy, and (2) society recognizes that expectation as objectively reasonable. Both prongs must be met for Fourth Amendment protections to apply.

According to United States v. Mendenhall, a seizure of a person occurs when, by means of physical force or a show of authority, an individual's freedom of movement is restrained, and a reasonable person would believe they were not free to leave due to police conduct.

A police officer may conduct a "Terry stop" and frisk when they have reasonable suspicion that criminal activity is afoot and that the individual is armed and dangerous. This limited pat-down is specifically for weapons to ensure officer safety.

For a valid search warrant, it must be (1) based on probable cause, (2) supported by an oath or affirmation, and (3) particularly describe the place to be searched and the items or persons to be seized.

Illinois v. Gates replaced the rigid two-pronged test from previous cases with a "totality-of-the-circumstances" approach for determining probable cause. This allows judges to consider all relevant factors, including the reliability of an informant's tip, in a more flexible manner.

The "automobile exception" allows police to search a vehicle without a warrant if they have probable cause to believe it contains contraband or evidence of a crime. The rationale is based on the vehicle's inherent mobility, which makes obtaining a warrant impractical, and a reduced expectation of privacy in vehicles.

The primary purpose of the exclusionary rule is to deter unlawful police conduct by making evidence obtained in violation of the Fourth Amendment generally inadmissible in criminal proceedings. Mapp v. Ohio extended this rule to the states.

Two exceptions to the exclusionary rule are:

Good Faith Exception: Evidence obtained by officers acting in reasonable reliance on a facially valid warrant, later found to be defective, may still be admitted if the officers acted in good faith.

Inevitable Discovery: Evidence will not be excluded if the prosecution can demonstrate it would have been inevitably discovered through lawful means, regardless of the initial unlawful search.

Riley v. California held that police generally need a warrant to search the digital contents of a cell phone seized incident to arrest, recognizing the vast amount of personal data on modern devices. Carpenter v. United States ruled that accessing historical cell site location information (CSLI) without a warrant violates the Fourth Amendment, reining in the third-party doctrine for comprehensive digital location data.