ABSTRACT Globally and in Vietnam, people who inject drugs (PWID) are disproportionately impacted by the HIV epidemic. PWID have considerably lower levels of care engagement and experience worse HIV outcomes than individuals with HIV who do not inject drugs. Efforts to scale-up HIV testing and treatment have largely failed to reach PWID, who experience unique barriers to care. Even fewer of these evidence-based interventions (EBIs) have been normalized (the process by which an intervention is integrated into routine clinic care) and sustained, resulting in significant losses of time and resources. The success of EBI implementation and sustainment is affected by clinic characteristics, which can act as important implementation barriers or facilitators. However, the ways in which clinic characteristics are related to EBI fit, normalization, and sustainment are poorly understood. The proposed fellowship builds on the applicant’s prior background to prepare her for a career as an independent mixed methods researcher and implementation scientist. The specific fellowship goals are for the applicant to 1) develop a strong foundation in conducting HIV-related implementation science research; 2) build advanced quantitative analysis skills in analyzing and interpreting statistical models; 3) expand her expertise in conducting mixed methods research and analysis; and 4) deepen her skills in presenting research findings and grantsmanship. These goals will be accomplished through coursework, mentorship from the sponsor and mentorship team, and execution of the proposed research project. The proposed mixed methods research will be completed within the context of a larger NIDA-funded cluster randomized implementation trial with 42 HIV testing sites in 10 provinces in Vietnam (Scaling up HPTN 074: a Cluster Randomized Implementation Trial of an EBI for Antiretroviral Therapy for PWID in Vietnam, 5R01DA047876-02). This study will identify key clinic characteristics for scale-up and sustainment of the systems navigation and psychosocial counseling (SNaP) EBI in Vietnam, and examine how adaptations to both the intervention and implementation strategies influence sustainment of SNaP. Aim 1 will use qualitative interviews conducted with participant PWID (n=36) and clinic staff (n=24-36), survey data collected at baseline and endline, and the modified Delphi method (to improve scale-up measurement) with US and Vietnamese experts in HIV and implementation science to identify factors influencing clinic scale-up success of SNaP. Aim 2 will use clinic staff surveys across the 42 sites to evaluate sustainment of SNaP and clinic-level factors associated with sustainment. Aim 3 will involve 24-36 qualitative interviews with clinic staff and directors to determine how intervention and implementation strategy adaptations and clinic characteristics influence SNaP normalization and sustainment. The proposed research aligns with NIDA’s goals of bridging the research-to- practice g...