This issue is addressed by looking at the effects of the abolition of the borstal training system and introduction of youth custody brought about by the Criminal Justice Act of 1982. This act contained changes in the ideology and structure of custodial sentencing of young adult offenders with consequential implications for the period of compulsory post-release supervision to which this age group is subject. A historical analysis of the shift in sentencing ideology shows the removal of rehabilitation from the goal of custodial arrangements, which leaves ambiguity as to the real purpose of post-release supervision. The author uses an exploratory study of probation officers' responses to the change to examine the difficulties of community-based supervision of young adults as they are affected by ambiguity about the justification in principle of the post-release supervision. The characteristics of this offender group make them particularly vulnerable to punitive, rather than rehabilitative measures, and the lack of explicitly rehabilitative goals tends to inhibit the retention and development of rehabilitative methods in supervision. 2 tables, 21 references.