"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 person or things to be seized."
The Fourth Amendment of the United States Constitution, "search and seizure law," protects the citizen's right to privacy and places limitations on the power of police to make arrests, search people and seize physical items or contraband. Therefore, the 4th Amendment attaches to the citizen either before an arrest has been made or shortly thereafter.
Implicit in the 4th Amendment is that some searches are reasonable and may be permitted under certain circumstances. These are where...
(1) the police have probable cause (a reasonable and articulable belief) to believe they can find evidence that you committed a crime, and a judge issues a search warrant based on a police affidavit, or
(2) the particular nature of the offense or circumstances surrounding the offense justifies a search without a warrant in order to protect the public or law enforcement, or because there is a diminished expectation of privacy. Examples of such circumstances include: exigent circumstances (imminent threat to life, serious threat to property, escape or destruction of evidence) or the citizen is in a forum in which it is unreasonable to believe he had any privacy right or expectation (e.g. a person growing illegal drugs in an unfenced backyard).