Parent training for foster parents is mandated by federal law and supported by state statues in nearly all states; however, little is known about the efficacy of that training, and recent reviews underscore that the most widely used curricula in the child welfare system (CWS) have virtually no empirical support (Grimm, Youth Law News, April-June:3-29, 2003). On the other hand, numerous theoretically based, developmentally sensitive parent training interventions have been found to be effective in experimental clinical and prevention intervention trials (e.g., Kazdin and Wassell, Journal of the American Academy of Child and Adolescent Psychiatry, 39:414-420, 2000; McMahon and Forehand, Helping the noncompliant child, Guilford Press, New York, USA, 2003; Patterson and Forgatch, Parents and adolescents: I. Living together, Castalia Publishing, Eugene, OR, USA, 1987; Webster-Stratton et al., Journal of Clinical Child Pyschology Psychiatry, 42:943-952, 2001). One of these, Multidimensional Treatment Foster Care (MTFC; Chamberlain, Treating chronic juvenile offenders: Advances made through the Oregon Multidimensional Treatment Foster Care model, American Psychological Association, Washington, DC, USA, 2003), has been used with foster parents of youth referred from juvenile justice. The effectiveness of a universal intervention, KEEP (Keeping Foster Parents Trained and Supported) based on MTFC (but less intensive) was tested in a universal randomized trial with 700 foster and kinship parents in the San Diego County CWS. The goal of the intervention was to reduce child problem behaviors through strengthening foster parents' skills. The trial was designed to examine effects on both child behavior and parenting practices, allowing for specific assessment of the extent to which improvements in child behavior were mediated by the parenting practices targeted in the intervention. Child behavior problems were reduced significantly more in the intervention condition than in the control condition, and specific parenting practices were found to mediate these reductions, especially for high-risk children in foster families reporting more than six behavior problems per day at baseline.