BELONGING TO A PEER GROUP IS USUALLY A POSITIVE FACTOR IN CHILDREN'S DEVELOPMENT, SINCE IN SUCH A GROUP THEY LEARN THE RULES OF SOCIAL BEHAVIOR AND SATISFY THEIR NEED TO BELONG, TO BE A PART OF SOCIETY. HOWEVER, SOMETIMES COLLECTIVE DELINQUENCY, SUCH AS PETTY THEFT OR HOOLIGANISM (RUFFIANLY BEHAVIOR< TAKES PLACE IN THESE GROUPS. THIS PROBLEM EMERGES ONLY WHEN THE DELINQUENCY BECOMES SYSTEMATIC, AND WHEN IT IS THE GROUP'S MAIN OBJECTIVE. FOUR FACTORS ARE DECISIVE IN TURNING A PEER GROUP INTO A GANG OF DELINQUENTS: THE GROUP'S ACCEPTANCE OF THE CRIMINAL SUBCULTURE WITH WHICH THEY HAVE COME IN CONTACT, LACK OF PARENTAL OR SCHOOL CONTROL OVER THE GROUP, LACK OF AFFILIATION TO OTHER SOCIAL GROUPS (ESPECIALLY FAMILY AND SCHOOL), AND LACK OF SOCIALLY ACCEPTABLE BEHAVIORAL PATTERNS. THE RESULTS OF ONE POLISH STUDY HAVE SHOWN THAT 76 PERCENT OF JUVENILE DELINQUENTS SPENT THEIR FREE TIME IN GROUPS WHICH WERE NOT CONTROLLED, WHILE ONLY 8 PERCENT OF NONDELINQUENT CHILDREN SPENT THEIR FREE TIME IN UNCONTROLLED GROUPS. WEAKENING OF THE FAMILY BONDS, ESPECIALLY IN THE CITIES, AND LONGER PERIODS OF SCHOOLING REQUIRED FOR HIGHER JOB QUALIFICATIONS (DUE TO INDUSTRIALIZATION) TEND TO ISOLATE THE YOUNG AND INCREASE THEIR RELIANCE ON PEER GROUPS. THE ORGANIZATION OF FREE TIME FOR YOUNG PEOPLE IS THEREFORE ESSENTIAL FOR JUVENILE DELINQUENCY PREVENTION. FOOTNOTES ARE INCLUDED. --IN POLISH.