After school programs (ASPs) are popular and receive substantial public funding. Aside from their child-care and supervision value, ASPs often provide youth development and skill-building activities that might reduce delinquent behavior. These possibilities and the observation that arrests for juvenile crime peak between 2 p.m. and 6 p.m. on school days have increased interest in the delinquency prevention potential of ASPs. This study examined effects of participation in ASPs conducted in Maryland during the 1999--2000 school year and the mechanism through which such programs may affect delinquent behavior. Results imply that participation reduced delinquent behavior for middle-school but not for elementary-school-aged youths. This reduction was not achieved by decreasing time spent unsupervised or by increasing involvement in constructive activities, but by increasing intentions not to use drugs and positive peer associations. Effects on these outcomes were strongest in programs that incorporated a high emphasis on social skills and character development.