PROJECT SUMMARY Child mental, emotional, and behavioral disorders are a critical public health problem, affecting between 13%- 20% of U.S. children. A barrier constraining the ability of existing interventions to more effectively combat these disorders is a lack of understanding regarding how preventive interventions achieve positive change. This project seeks to use existing data from a landmark effectiveness trial of an evidence-based parenting intervention to determine the intervention components most functional in achieving positive outcomes. The proposed R03 project will accomplish the following specific aims: 1) Develop an observational coding system capable of measuring fidelity to specific core components of the evidence-based PMTO parenting intervention; 2) Empirically determine the extent to which each observed PMTO core component is associated with changes in parenting following intervention exposure; and 3) Examine associations between observed PMTO core components and child outcomes (i.e., internalizing, externalizing) from baseline to 9-year follow-up, mediated by changes in parenting. A five-step process will be employed to systematically develop an observational coding system using video data depicting weekly PMTO parenting group sessions. Multilevel modeling will then be employed to determine the extent to which each observed PMTO core component is associated with child outcomes over time, with changes in parenting included in the model as a mediator. A better understanding of the intervention components most functional in achieving positive outcomes can advance intervention research and improve clinical practice by enabling interventions to be delivered more efficiently in real-world settings. These efforts can ultimately improve the prevention of child mental, emotional, and behavioral problems.