Government Action and Reasonable Expectation of Privacy
The Fourth Amendment protects against unreasonable search and seizure, but only searches conducted by government agents. Private citizens conducting searches (even illegally) do not implicate it. Once government action is established, the court asks whether the person had a reasonable expectation of privacy in the place searched. This is a fact-sensitive analysis: you have one in your home, your car, and your backpack, but not in an open field visible from the street, or on a trash can left at the curb. The reasonable expectation test, from Katz v. United States, replaced a purely property-based view and focuses on what society recognizes as a legitimate privacy interest.
Warrant Requirement and Exceptions
If the search violates a reasonable expectation of privacy, a warrant is normally required. The warrant must be supported by probable cause (a fair probability that contraband or evidence of crime is at the location) and issued by a neutral magistrate. But exceptions abound: searches incident to arrest, plain view doctrine, consent, exigent circumstances (emergency), vehicle searches, and administrative searches. These exceptions allow searches without warrants when magistrate approval is impractical. Evidence obtained in violation of the Fourth Amendment is excludable under the exclusionary rule, though not in all contexts (impeachment, immigration proceedings).