ALTHOUGH POLICE LONG HAVE BEEN WORKING UNDERCOVER IN SUCH SPECIAL PROBLEM AREAS AS NARCOTICS AND ORGANIZED CRIME, IT WAS ONLY IN THE LATE 1960'S THAT PLAINCLOTHES OFFICERS TURNED THEIR ATTENTION TO ORDINARY STREET CRIMINALS. THE FIRST FORMAL STREET CRIME UNIT EVOLVED FROM NEW YORK CITY'S TACTICAL PATROL FORCE ASSIGNED TO GHETTO RIOTS AND ANTIWAR DEMONSTRATIONS. THE NEW YORK UNIT EVENTUALLY GREW TO 300 OFFICERS AND BECAME A MODEL FOR SIMILAR PROGRAMS IN CITIES THROUGHOUT THE COUNTRY. POLICE ENTHUSIASTICALLY ENDORSE THE CONCEPT BEHIND STREET CRIME UNITS, CITING THOUSANDS OF ARRESTS AND HIGH CONVICTION RATES FOR STREET CRIME UNITS. PROSECUTORS ARE SOMEWHAT LESS ENTHUSIASTIC, NOTING THAT THE UNITS APPREHEND ONLY PETTY THIEVES, NOT SERIOUS CRIMINALS. PUBLIC DEFENDERS QUESTION THE QUALITY AND OFTEN THE PURPOSE OF THE UNITS. MOST STREET CRIME UNITS USE A MIX OF PLAINCLOTHES STREET SURVEILLANCE AND DECOY TACTICS. OFFICERS EITHER ATTEMPT TO BLEND IN WITH THE CITIZENRY TO PROVIDE THEMSELVES WITH THE NECESSARY ANONYMITY AND FREEDOM OF MOVEMENT TO PURSUE SUSPECTS UNDETECTED, OR PRESENT THEMSELVES AS HIGHLY VULNERABLE POTENTIAL VICTIMS. THE DETAILS OF THE NEW YORK STREET CRIME OPERATION ARE PRESENTED, TOGETHER WITH BRIEF DESCRIPTIONS OF STREET CRIME UNITS IN SEATTLE, DENVER, AND OTHER CITIES (INCLUDING DETROIT, WHERE A STREET CRIME UNIT FAILED). THE MAJOR FINDINGS OF EVALUATIONS OF THE NEW YORK AND DENVER OPERATIONS ARE CITED. A SIDEBAR TO THE ARTICLE DESCRIBES ONE NIGHT'S ACTIVITIES OF A NEW YORK STREET CRIME UNIT PATROL. (LKM)