Can police pat you down without charging you?

A law enforcement officer may perform a 'pat down' on the outer clothing of an individual when performing a stop for investigation on the street, if the peace officer feels that their safety may be is in jeopardy.

The officer may also perform a 'pat down' if he/she has reasonable suspicion of the individual's immediate criminal intent and/or the individual's involvement with a past criminal activity; (the individual may match a suspect's description, or the individual may be in the vicinity of a recent crime scene, and the peace officer has reasonable suspicion of the individual's involvement).

Added: This is referred to as a "Terry stop" referring to the US Supreme Court case "Terry v. Ohio" which ruled that such a stop and frisk IS Constitutional.