http://criminal.findlaw.com/criminal-rights/searches-and-seizures-the-limitations-of-the-police.html
Under the Fourth Amendment to the U.S. Constitution, police may engage in "reasonable" searches and seizures.
To prove that a search is "reasonable," the police must generally show that it is more likely than not that a crime has occurred, and that if a search is conducted it is probable that they will find either stolen goods or evidence of the crime. This is called probable cause.
In some situations, the police must first make this showing to a judge who issues a search warrant. In many special circumstances, however, the police may be able to conduct a search without a warrant. In fact, the majority of searches are "warrantless."
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