This is a multi-method, multiple cohort, RCT for an early childhood school-based intervention designed to decrease multiple subtypes of aggression and peer victimization and facilitate social and emotional learning, which has clear theoretical, educational, clinical, and policy implications. Based on rigorous prior theory and research including extensive preliminary data, we propose to assess the impact of the Early Childhood Friendship Project (ECFP) on changes in aggression/peer victimization subtypes, prosocial behavior, and social and academic competence with a teacher-implemented (with coaching) version of the program. Further, we will examine whether changes in executive functioning, emotion regulation, and hostile attribution biases indirectly account for the program effects. We will test if physiological reactivity (skin conductance and respiratory sinus arrhythmia) serves as moderators of intervention effects. Data will be collected from 600 children (30 randomly assigned preschool classrooms) diverse in SES and race/ethnicity. We will use multiple methods (school-based observations, direct academic assessments, child interviews, physiological reactivity using two tasks, observer, caregiver, and teacher reports) to assess the efficacy of the program, hypothesized mechanisms, and role of physiology as a moderator of intervention effects. The duration of the effects will be tested at both 4 month and 12-month follow-up and will thus demonstrate the impact the program has on children’s school readiness and transition to kindergarten. The current proposal will use time intensive and state-of-the-art techniques for assessing all constructs, which enhances the rigor of the approach. Investigating the effects of a teacher-coaching model of the ECFP provides a critical next step in large-scale dissemination of the program. Additionally, focusing on the role of physiological reactivity is a key goal for clinical child research on the biological embedding of early adversity and its consequences prior to and after the transition to school, which supports the innovation and impact of the proposal. Understanding these mechanisms implicated in the efficacy of the program will enhance our understanding of how to foster wellness and school readiness. The proposed project will provide a rich dataset with opportunities for secondary questions not contingent on the efficacy of the ECFP as well as additional exploratory analyses related to functions of aggression and bullying subtypes. The proposed project is well positioned to advance major “high-priority” initiatives of the Eunice Kennedy Shriver National Institute of Child Health and Human Development (Child Development and Behavior Branch): Advancing research on the social, environmental, and biological factors that impact adaptive behavior development and school functioning. In addition, this work is related to NICHD’s “high-priority” research areas such as: psychosocial adjustment for individuals in h...