Jack Tizard's belief that greater improvements in performance and behavior are likely to attend efforts to change environments rather than inner dispositions is supported by 20 years of Home Office research into delinquency prevention. Situational measures which reduce opportunities for crime seem more effective than attempts to reform delinquents. Given that most delinquents desist from crime in early adulthood, minimal interference in their lives may be the best policy. Humane care rather than reform should be the goal for those requiring institutional placement. 4 tables and 81 references. (Author abstract modified)