The Fourth Amendment of the U.S. Constitution provides, "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." Ultimately, these words endeavor to protect two fundamental liberty interests - the right to privacy and freedom from arbitrary invasions.

A search occurs when an expectation of privacy that society considers reasonable is infringed by a governmental employee or by an agent of the government. Private individuals who are not acting in either capacity are exempt from the Fourth Amendment prohibitions.

A seizure refers to the interference with an individual's possessory interest in property. To meet the definition of an unreasonable seizure, the property's owner must have had a reasonable expectation of privacy in the items seized. A person is seized when law enforcement personnel use physical force to restrain the person if a reasonable person in the same or a similar situation would not feel free to leave the situation. The previous owner of abandoned property cannot allege an unreasonable seizure of that abandoned property. Abandoned property is property left behind by its owner in a manner in which the owner abandons the possessory interest in the property and no longer retains a reasonable expectation of privacy with regard to the search.

The prohibition on unreasonable searches and seizures particularly affects the work of law enforcement personnel by restricting the actions that they may take in performing a criminal investigation; however, the ban also disallows unreasonable searches and seizures in the civil litigation context. Law enforcement may only conduct a search if individualized suspicion motivates the search. The Fourth Amendment prohibits generalized searches, unless extraordinary circumstances place the general public in danger.

To sue regarding an alleged Fourth Amendment violation, the plaintiff must have standing. Standing with respect to Fourth Amendment violations requires that the plaintiff have had a legitimate expectation of privacy at the searched location. A legitimate expectation of privacy must meet both the subjective and objective tests of reasonableness. The subjective test requires that the plaintiff actually and genuinely expected privacy, and the objective test requires that given the circumstances, a reasonable person in the same or a similar situation would have expected privacy as well.

The Fourteenth Amendment of the U.S. Constitution applies the Fourth Amendment's provisions against the states as well as the federal government. See Mapp v. Ohio, 367 U.S. 643 (1961).

Courts ordinarily suppress evidence obtained during an unreasonable search or seizure and offered against the accused. See Mapp v. Ohio, 367 U.S. 643 (1961). This rule, known as the exclusionary rule, applies equally to both the investigatory and accusatory stages of a criminal prosecution.

In order to avoid illegally searching or seizing the property of a suspect, law enforcement personnel typically obtain search warrants. To obtain a search warrant, law enforcement must show probable cause, must support the showing by oath or affirmation, and must describe in particularity the place they will search and the items they will seize. A judge can find probable cause only be examining the totality of the circumstances.

Different types of warrants exist. A knock-and-announce warrant requires law enforcement personnel to knock on the door of a residence and announce their identity before entering, giving the owner or occupier an opportunity to answer the door. In 2006, the U.S. Supreme Court determined that law enforcement's failure to knock or announce when in possession of a knock-and-announce warrant does not necessitate use of the exclusionary rule. See Hudson v. Michigan, 547 U.S. 586 (2006).

No knock warrants allow law enforcement personnel to enter a building or home without announcing their presence and without knocking on the door first. Courts reserve these warrants for situations in which a building's owner or occupier could destroy the sought-after evidence by the time law enforcement waits for the owner or occupier to open the door.

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