The Fourth Amendment to the United States Constitution is "designed 'to prevent arbitrary and oppressive interference by enforcement officials with the privacy and personal security of individuals." The Amendment is currently interpreted as consisting of two separate clauses, the first generally prohibiting unreasonable searches and seizures, and the second requiring the establishment of probable cause prior to the issuance of a warrant. Hence, only those government searches and seizures requiring a warrant necessitate the establishment of probable cause, and all other searches and seizures simply need to be "reasonable."
Margaret A. Hoehl,
Usual Suspects Beware: "Walk, Don't Run" Through Dangerous Neighborhoods,
U. Rich. L. Rev.
Available at: http://scholarship.richmond.edu/lawreview/vol35/iss1/6