Parent-Child Interaction Therapy (PCIT) is an empirically validated treatment for externalizing behavior problems in young children. Foster children are at an especially high risk of developing externalizing problems, difficulties that their foster parents are not often trained to manage effectively. PCIT is typically delivered by therapists in 12-14 individual, weekly sessions. Although efficacious, standard PCIT is resource intensive, so it is unlikely to be adopted as a routine service by child welfare agencies. Therefore, the objective of this study is to implement and test the efficacy of a novel adaptation of PCIT with foster families using group-based and in- home treatment. The intervention will couple two innovative treatment modalities: (1) an intensive PCIT group training with foster parent-child dyads, and (2) daily in-home rehearsal of PCIT skills reinforced by regular telephone counseling sessions. A total of 132 foster families will be randomly assigned to three equal groups (n=44), including two experimental conditions receiving different treatment doses. Both treatment groups will receive a two-day training followed by 8 weeks of in-home clinical services. One of the two treatment groups will then receive a one-day booster session of PCIT training along with 6 more weeks of in-home services. The two experimental groups will be compared to each other and a third waitlist control group. The study has three central aims: (a) to examine whether two treatment conditions receiving different doses of PCIT demonstrate gains in parent and child outcomes compared to each other and to a control group receiving child welfare services as usual, (b) to identify predictors of treatment non-response, and (c) to explore mediators of main- effect findings. Assessments will be conducted at baseline and several subsequent time points for each group. Multiple informants and measures will be used to assess study outcomes, which include children's externalizing behaviors, compliance, and emotion regulation as well as caregivers'behavior management skills, parenting attitudes and stress. This investigation will make several significant practical and scientific contributions. Analyses will show whether PCIT can be implemented successfully in a community setting with foster families using group-based training and follow-up in-home activities. Through the use of the same therapeutic modalities across treatment conditions, it will also be possible to detect dosage effects. Furthermore, the proposed intervention builds on existing child welfare service structures, thereby increasing the likelihood of successful (a) implementation of the intervention, (b) replication in future effectiveness trials, and (c) integration into the child welfare service system. Methodologically, the use of a randomized, controlled design along with psychometrically validated measures, triangulated assessments, and fidelity safeguards will increase internal validity and strengthen statistical conclusions. Likewise, the proposed analytic approach will enhance causal inferences and contribute to future program development and design.111 PUBLIC HEALTH RELEVANCE: Young foster children are at a high risk of externalizing problems, difficulties that foster parents typically are not trained to address with evidence-based behavior management techniques. This study will evaluate an innovative adaptation of Parent-Child Interaction Therapy (PCIT) with foster children and their caregivers using a group-based approach reinforced with in-home training. Results will produce new knowledge about the feasibility of translating PCIT, an empirically validated clinical treatment, into a community setting in order to mitigate externalizing problems in young foster children.