Project Summary/Abstract Youth in foster care are a vulnerable group of adolescents at high risk of acquiring sexually transmitted infections (STIs), including HIV. Youth in foster care are estimated to have 3 times the risk of a diagnosed STI compared to youth not in care. The unique developmental experience of youth in foster care–marked by a challenging social context including instability, disrupted attachments with caregivers, and adverse childhood experiences–contributes to these health disparities. This social context, along with the neurodevelopment and biological changes common during adolescence, place foster youth at particularly high risk of acquiring HIV/STIs. Developmentally tailored HIV/STI prevention programs can reduce this risk, therefore access to such programs is critical for preventing HIV/STIs among youth in foster care. However, due to their transient social context, foster youth lack access to consistent and accurate HIV/STI prevention information. The purpose of this study is to address this gap in access to a tailored HIV/STI prevention program for foster youth by: (1) adapting an evidence-based eHealth HIV/STI prevention program for use with youth in foster care (Research Aims 1 and 2), (2) identifying possible barriers and facilitators to the program’s implementation (Research Aim 3), and (3) providing training to facilitate the proposed F31 PI’s ultimate career goal of becoming an independent research scientist in HIV prevention. The proposed study methods are grounded in the Consolidated Framework for Implementation Research, which will be used to inform a mixed methods approach to program adaptation, usability testing, and identification of barriers and facilitators to implementation. To adapt the eHealth HIV/STI prevention intervention, this study will utilize systematic, participatory methods engaging a Youth Advisory Board to incorporate developmentally appropriate adaptations. To elucidate individual- and inner-level barriers and facilitators to program implementation, this study will use questionnaires to sample key stakeholders (e.g., social workers, foster youth, foster parents) affiliated with the foster care system. This study will fill a gap in access to evidence-based eHealth HIV/STI prevention programs for a marginalized and mobile population of youth at heightened risk of HIV/STI acquisition. This proposed study implements the goals of NOT-MH-20-035, “Developmentally Tailored HIV Prevention and Care Research for Adolescents and Young Adults,” by tailoring an HIV/STI prevention program for youth in foster care and conducting implementation science to identify barriers and facilitators to program implementation. Through conducting this F31 study in coordination with proposed formal training activities, the proposed F31 PI will gain rigorous training towards becoming an independent research scientist in the following areas: (1) implementation science research methods; (2) qualitative content analysis; (3) ...