PROJECT SUMMARY Various barriers including stigma can limit access to HIV and substance use for people with HIV who inject drugs. Acceptance and Commitment Treatment (ACT) empowers these people to cope with stigma and other care barriers, leading to an increase in health services utilization. ACT is an evidence-based intervention that uses acceptance, mindfulness, and behavior change processes to improve psychological flexibility. In the SCRIPT randomized controlled trial, we showed that ACT could increase antiretroviral therapy (ART) initiation and substance use care engagement for people with HIV and active injection drug use. This study recruited participants at a community-based harm reduction site and trained psychologists delivered the intervention at a community clinic, but community-engaged implementation strategies have not yet been studied. Implementation strategies for evidence-based care access interventions for people with HIV and injection drug use are rare. This project will narrow this gap, assessing appropriate implementation outcomes and their relations with trial outcomes. We will also explore implementation strategies, to be tested in future hybrid effectiveness-implementation trials of ACT. Using the existing mixed-methods data from the SCRIPT trial, this R03 proposes to assess implementation outcomes and identify implementation strategies (including intervention adaptation, training, fidelity monitoring). The results from this project will aid in access to health services for the stigmatized population of people with HIV who inject drugs and provide the groundwork for future trials testing acceptance-based approaches to improve care access for people with HIV, substance use and potentially other stigmatized conditions.