Project Abstract Evaluation of the school-based Healthy Relationships Project for primary prevention of child sexual abuse among children pre-K through 5th grade Child sexual abuse (CSA) is a major issue worldwide, with rigorous evidence of its magnitude and impacts on social, emotional and physical health. While interventions for the primary prevention of CSA exist, few have been rigorously evaluated. A review of school-based education programs for CSA prevention (Walsh et al., 2015) found a total of 24 trials representing 5,802 participants in both elementary and high schools across 7 countries. Findings included evidence of improvements in protective behaviors and knowledge, with limited evidence of increased odds of disclosure. None found changes in incidence/prevalence of CSA over time. The Healthy Relationships Project (HRP) created, modified and run by the organization Prevent Child Abuse Vermont, has been delivering CSA primary prevention curricula since 1990 with implementation across 30 U.S. States, including statewide in Vermont. During those decades, substantiated cases per year in Vermont dropped 61% and the number of child perpetrators per year dropped 69%. Addressing Research Priority 1, implementation of the HRP in Pre-K through 5th grade will be rigorously evaluated in a mixed-methods stepped wedge randomized trial design with 16 public charter schools in three high-need wards in Washington, DC. The School Safety Omnibus Amendment Act of 2018 was enacted to obligate DC schools to prevent and address student sexual abuse. Our partner Safe Shores, the Child Advocacy Center (CAC) for DC, has implemented HRP in public charter schools with success; the opportunity remains to conduct this trial in schools where it has not been delivered and increase its feasibility via established trust with the district and schools. Our findings about its efficacy will potentially place the HRP into the echelon of evidence-based programs that many schools seek to use, especially those mandated to deliver CSA prevention programming. Data from Safe Shores, the CAC that receives reports of suspected child abuse from all Washington, DC public schools, will be utilized as the primary outcome including the numbers and types of CSA investigations and other services provided by the CAC involving the participating schools which include forensic services, family advocacy, case management and more. More commonly utilized measures of improvements in protective behaviors, self- efficacy, knowledge and disclosure will be collected from caregivers (n=3,500) and teachers (n= 266). Qualitative research will be included to assess strengths and weaknesses of intervention rollout, fidelity monitoring, lessons learned and sustainability. This innovative and novel mixed methods evaluation study will move the science of child sexual abuse prevention research forward with a community-based participatory research partnership between scientists in family violence prevention research, professionals at community-based organizations and participating public elementary schools in high need urban areas. In summary, a rigorous experimental evaluation will be conducted of an existing universal primary prevention program with a history of 30+ years of implementation across 30 states and robust pilot data.