In the US, searches and arrests are generally unconstitutional unless they are authorized by warrant requiring probable cause or are otherwise “reasonable.”

The Fourth Amendment grants this right with the following language:

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.

Courts enforce this right by rendering the fruit of any unlawful search or seizure inadmissible at trial.  Ideally, knowing that evidence obtained by unlawful means will not be admissible at trial, police will not investigate crime in unlawful ways.  Unfortunately, as indicated by the volume of Fourth Amendment litigation, this is not always the case.  Being applied by trial courts after the police investigation is over, sympathy is not always with the criminal defendant, and on criminal appeals, courts give wide deference to the trial court ruling allowing only “clear error” review.

Practically speaking though, a criminal defense attorney’s arguments for the unconstitutionality of an investigation, especially in drug crimes, can be a strong tool for the defendant.

What constitutes an unreasonable search or seizure, however, is a complex matter.

While the Fourth Amendment generally allows properly warranted searches, most searches these days are not conducted pursuant to a warrant.  The constitutional support for these searches stem for numerous exceptions to the warrant requirement:

• Consent – Surprisingly often, individuals will consent to a search.  Individuals are under no obligation to allow the police to search their person or their belongings.  Also often, persons other than the defendant will grant consent to search the defendant’s belongings.
• Traffic stops –  Traffic stops require only reasonable, articulable suspicion that a law is being broken.
• License checks – License checks have been ruled constitutional as long as they meet certain requirements of the case law.
• Stop and Frisks – If an officer believes that criminal activity is afoot and that a suspicious person in armed,  and has reasonable, articulable suspicion that the person poses a danger to officers or others, the officer may frisk (pat-down search) the person for weapons.
• Plain-view Exception, or No legitimate expectation of privacy – If an item that invokes probable cause is in plain view from a public space, e.g. a bag of cocaine in a front yard adjoining a street, the police can search or seize it.  Police are also entitled to smell odors that they can smell from public spaces.  This doctrine also yields a plain-feel exception when coupled with the stop and frisk exception: when police must search a person’s clothing above the cloth, they may search a pocket upon plainly feeling an object whose feel indicates that it is contraband or a weapon, e.g. a gun or distinctively shaped piece of drug paraphernalia.
• Hot Pursuit – Law enforcement can follow someone they are validly following into a space that they would otherwise not be allowed to search and are not required to keep their eyes shut to evidence as they do so.
• Searches incident to valid arrests
• Protective sweeps – If police are already lawfully in a space (e.g. on authorization of an arrest warrant), and reasonably believe there may be danger, they may search the adjoining areas to ensure that no dangerous person is around and no one dangerous has access to weapons.
• Imminent Destruction of Evidence – If law enforcement can articulate a reasonable belief that evidence will be destroyed, they will be allowed to search as is necessary to recover the evidence before it is destroyed without a warrant.
• Public Safety – If the police have reasonable suspicion that a person or thing poses a danger to the public, they can search or seize the person or thing without warrant.
• Inventory Search – Upon taking possession of a person or a thing, law enforcement is entitled to take an inventory.  With arrested persons, they will be entitled to search the person.  With impounded items, like cars, they will be entitled to search the vehicle.