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A search warrant is a court order issued by a magistrate, judge or Supreme Court official that authorizes law enforcement officers to conduct a search of a person, location, or vehicle for evidence of a crime and to confiscate evidence if it is found. A search warrant cannot be issued in aid of civil process.
Jurisdictions that respect the rule of law and a right to privacy put constraints on the powers of police investigators, and typically require search warrants, or an equivalent procedure, for searches conducted as part of a criminal investigation. An exception is usually made for hot pursuit: if a criminal flees the scene of a crime and the police officer follows him, the officer has the right to enter a property in which the criminal has sought shelter. Conversely, in authoritarian regimes, the police typically have the right to search property and people without having to provide justification, or without having to secure the permission of a court.
United Kingdom 
Search warrants are issued by a local Magistrate and require a Constable to provide evidence to support the warrant application. In the vast majority of cases where the police already hold someone in custody, searches of premises[clarification needed] can be made without a search warrant under Section 18 of the Police and Criminal Evidence Act (PACE), which requires only the authority of a Police Inspector.
Searches under Section 18 Police and Criminal Evidence Act can be conducted immediately by a Constable without the requirement for an Inspector's authorisation under Section 18(5)a of PACE. This subsection allows a Constable to search the address of a suspect(s) under arrest in their presence before being presented to a police station (or other custody suite).
If a person is arrested on their own property or just after leaving their premises, a Constable may immediately search both them and the immediate area where the person was under Section 32 of PACE.
United States 
Under the Fourth Amendment to the United States Constitution, most searches by the police require a search warrant based on probable cause, although there are exceptions. Any police entry of an individual's home always requires a warrant (for either search or arrest), absent exigent circumstances, or the free and voluntary consent of a person with reasonably apparent use of or control over the property.
Under the Fourth Amendment, searches must be reasonable and specific. This means that a search warrant must be specific as to the specified object to be searched for and the place to be searched. Other items, rooms, outbuildings, persons, vehicles, etc. may require additional search warrants.
To obtain a search warrant, an officer must first prove that probable cause exists before a magistrate or judge, based upon direct information (i.e. obtained by the officer's personal observation) or hearsay information. Hearsay information can even be obtained by oral testimony given over a telephone, or through an anonymous or confidential informant, so long as probable cause exists based on the totality of the circumstances. Both property and persons can be seized under a search warrant. The standard for a search warrant is lower than the quantum of proof required for a later conviction. The rationale is that the evidence that can be collected without a search warrant may not be sufficient to convict, but may be sufficient to suggest that enough evidence to convict could be found using the warrant.
U.S. police do not need a search warrant to search a vehicle they stop on the road or in a non-residential area if they have a probable cause to think that it may contain contraband or evidence of a crime. In that case, police may search the passenger compartment, trunk, and any containers inside the vehicle capable of holding the suspected article. By comparison, under Australian law, police can exhaustively search any vehicle on a public road, and any electronic devices therein (mobile phone, computer), without the responsible persons' permission, for evidence of criminal acts, with or without proof or suspicion of any kind.
Police do not need a search warrant, or even probable cause, to perform a limited search of a suspect's outer clothing for weapons, if police have a reasonable suspicion to justify the intrusion—a Terry "stop and frisk".
In the United States, the issue of federal warrants is determined under Title 18 of the United States Code. The law has been restated and extended under Rule 41 of the Federal Rules of Criminal Procedure. Federal search warrants may be prepared on Form AO 93, Search and Seizure Warrant. Each state also enacts its own laws governing the issuance of search warrants.
A customs or immigration officer of the United States is not required to have any warrant, reasonable suspicion, or consent to search persons, vehicles, baggage, or cargo that have border nexus; regardless of citizenship or origin. This is clearly stated in 19 U.S.C. 1467, 19 C.F.R. 162.6.
In certain cases a search warrant is not required, such as where consent is given by a person in control of the object or property to be searched. Some commonly cited exigent circumstances are: hot pursuit of a felon (to prevent a felon's escape or ability to harm others); imminent destruction of evidence before a warrant can be properly obtained; emergency searches (such as where someone is heard screaming for help inside a dwelling); or a search incident to arrest (to mitigate the risk of harm to the arresting officers specifically).
Another exception is when evidence is in plain view—if the officer is legitimately on the premises, his observation is from a legitimate vantage point, and it is immediately obvious that the evidence is contraband (for example, a cannabis cigarette on the front seat of a car while the officer has pulled the suspect over for a seat belt violation), the officer is within his right to seize the object in question.
When police arrest an individual shortly after exiting a vehicle, the police may conduct a full search of the suspect's person, any area within that person's immediate reach, and the passenger compartment of the vehicle which was recently occupied, for weapons or any other contraband. (However, a recent Supreme Court decision limits such searches to circumstances where the arrested person has the possibility of accessing the vehicle, or when the vehicle could contain evidence of the crime that the person is being arrested for.) If the subject is arrested in a home, police may search the room in which they were arrested, and conduct a "protective sweep" of the premises where there is reasonable suspicion that other individuals may be hiding. Searches are also allowed in emergency situations where the public is in danger.
With rented property, a landlord may refuse to allow law enforcement to search a tenant's apartment without a search warrant, and a warrant must be obtained using the same guidelines as if it was a tenant's own home. But in some jurisdictions, a hotel room may be searched with permission of the hotel's management but without permission from a guest and without a warrant.
See also 
- Arrest warrant
- National Security Letter
- No-knock warrant, to enter a property without occupant notification
- Sneak and peek warrant, to clandestinely look inside a property
- Sugar bowl (legal maxim)
- Writ of Assistance
- Farin (29 April 2010). "Breaking and metering". BBC Watchdog. Retrieved 27 October 2012. "British Gas are legally entitled to enter anyone's home if the householder hasn't paid the bill and won't let them in, but they need a warrant from a magistrate, who must be satisfied the information British Gas shows them is correct. In most cases, British Gas also need to have issued a prior warning."
- AO 93 (Rev. 12/09) Search and Seizure Warrant. Uscourts.gov.
- Groh v. Ramirez, 540 U.S. 551, 564-65 (2004)
- Arizona v. Gant, No. 07-542, Decided April 21, 2009: 
- http://www.encyclopedia.com/doc/1G1-98253659.html[dead link]