Detention Short of Arrest:
Stop-and-Frisk .--Arrests are subject to the requirements of the Fourth Amendment, but the courts have followed the common law in upholding the right of police officers to take a person into custody without a warrant if they have probable cause to believe that the person to be arrested has committed a felony or has committed a misdemeanor in their presence. 6 The probable cause is, of course, the same standard required to be met in the issuance of an arrest warrant, and must be satisfied by conditions existing prior to the policeman's stop, what is discovered thereafter not sufficing to establish retroactively reasonable cause. 7 There are, however, instances when a policeman's suspicions will have been aroused by someone's conduct or manner, but probable cause for placing such a person under arrest will be lacking. 8 In Terry v. Ohio, 9 the Court almost unanimously approved an on-the-street investigation by a police officer which involved ''patting down'' the subject of the investigation for weapons. - See more at:
http://constitution.findlaw.com/amendment4/annotation03.html#2