Abstract Worldwide, five million adolescents and young adults (AYA) are living with HIV. More than 90% of global HIV- related mortality among AYA occurs in sub-Saharan Africa. A limited but growing evidence base of interventions available for adolescents in sub-Saharan Africa highlight the importance of retention in care for improving treatment outcomes. To meet AYA where they are in their engagement with the healthcare system, interventions need to be feasible, acceptable, and appropriate for local healthcare settings and practice. This F31 project leverages a cluster randomized controlled trial, Data-informed Stepped Care to Improve Adolescent HIV Outcomes (UG3/UH3 HD096906; PIs: Kohler, John-Stewart) that is testing a Stepped Care intervention to improve retention among adolescents and young adults living with HIV in western Kenya. This F31 project aims to evaluate early phase implementation outcomes of the Stepped Care intervention as it is delivered across 10 healthcare facilities. Aim 1 will identify adaptations made to the intervention (materials, procedures, or delivery) that influence acceptability, feasibility, and appropriateness. The Framework for Reporting Adaptations and Modifications to Evidence-based interventions (FRAME) will be used to characterize adaptations and modifications necessary for facility-specific optimization of the intervention. Aim 2 will identify the barriers and facilitators that impact penetration, coverage, and fidelity of the Stepped Care intervention. We will apply the Consolidated Framework for Implementation Research (CFIR) to examine barriers and facilitators of implementation to understand the context and intervention characteristics influencing implementation of the Stepped Care intervention. Finally, Aim 3 will match barriers to successful implementation to targeted strategies to overcome them, producing a package of intervention implementation options that promote long-term intervention adoption and sustainability. Findings from this project will contribute to understanding optimization of Stepped Care and provide valuable information about the context necessary for successful scale-up of Stepped Care interventions for adolescents and young adults within Kenya or scale out to other global settings. The research plan will provide the F31 candidate, an implementation science pre-doctoral student, with rigorous training in (1) implementation science conceptual frameworks, models, design and methodology; (2) training in qualitative methods; and (3) content-area expertise regarding evidence-based interventions for adolescents and young adults living with HIV.