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 (Farmer et al., 2007; Grimm, 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 & Wassell, 2000; McMahon & Forehand, 2003; Patterson & Forgatch, 1987; Webster-Stratton et al., 2001). One of these, Multidimensional Treatment Foster Care (MTFC; Chamberlain 2003), has been used with foster parents of youth referred from juvenile justice. The effectiveness of an intervention based on MTFC 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 (i.e., those with more than 6 behavior problems per day at baseline).