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.
The purpose of the 4th Amendment is to protect people from being abused by a powerful government. Due to the thousands of daily governmental intrusions — such as airport checks, traffic stops, drug testing, obtaining of digital evidence, traditional criminal law enforcement practices and regulatory inspections — the Fourth Amendment is the most commonly implicated and litigated part of our Constitution.
The Fourth Amendment originally enforced the notion that “each man’s home is his castle,” secure from unreasonable searches and seizures of property by the government. It protects against arbitrary arrests, and is the basis of the law regarding search warrants, stop-and-frisk, safety inspections, wiretaps, and other forms of surveillance, as well as being central to many other criminal law topics and to privacy law.
Probable cause is a requirement that must usually be met before police make an arrest, conduct a search, or receive a warrant. Courts usually find probable cause when there is a reasonable basis for believing that a crime may have been committed (for an arrest) or when evidence of the crime is present in the place to be searched (for a search). Under exigent circumstances, probable cause can also justify a warrantless search or seizure. Persons arrested without a warrant are required to be brought before a competent authority shortly after the arrest for a prompt judicial determination of probable cause.
Contrary to popular belief, the right to privacy is not specifically mentioned in the U.S. Constitution. Over the years, the courts have interpreted the 4th Amendment, along with other Amendments such as the 9th, to protect privacy in many situations.
Most popular, perhaps, is the landmark case of Mapp v. Ohio (1961) – a landmark case in criminal procedure, in which the United States Supreme Court decided that evidence obtained in violation of the Fourth Amendment, which protects against “unreasonable searches and seizures,” may not be used in state law criminal prosecutions in state courts, as well as in federal criminal law prosecutions in federal courts as had previously been the law. The Supreme Court accomplished this by use of a principle known as selective incorporation; in Mapp this involved the incorporation of the provisions, as interpreted by the Court, of the Fourth Amendment which is applicable only to actions of the federal government into the Fourteenth Amendment due process clause which is applicable to actions of the states.