The Fourth Amendment is arguably the most important constitutional amendment in the context of criminal cases. It states that “[t]he right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated.”
To trigger Fourth Amendment protection, there must be a “search.” There are two categories of searches under the Fourth Amendment.
The first category of search is a physical intrusion of a person, house, paper, or effect by the government for the purpose of obtaining information. Stated otherwise, if the government commits a common-law trespass upon private property to look for something, then it is a search for purposes of the Fourth Amendment. Some examples include entering the home, looking through a person’s cell phone, extracting blood from a person, or placing a GSP tracking device on a vehicle.
The second category of search is the government’s violation of a person’s subjective expectation of privacy that society recognizes as reasonable. This is gleaned from the 1967 case of Katz v. the United States, which involved the installation of a listening device that was attached to the outside of a public telephone booth and that allowed police officers to eavesdrop on one end of the target's phone conversation. Instead of just a physical intrusion, the Katz reasonable expectation of privacy test provides that a search can occur by invading someone’s expectation of privacy while obtaining information through any of the five senses, most often through the sense of sound.
Many criminal cases are decided on whether an unreasonable search or seizure occurred under the Fourth Amendment.