PROJECT SUMMARY Background. Sexual and gender minorities (SGM) experience among the largest mental health disparities of any population, at substantial cost to society. One driver has been the lack of evidence-based practice (EBP) addressing the distinct mechanisms underlying SGM's risk. For 10+ years, our team has developed LGBTQ- affirmative cognitive-behavioral therapy (CBT) as the first and only EBP specifically for SGM mental health, now in high demand. This proposal seeks to study optimal means of implementing this effective treatment at LGBTQ community centers nationwide to advance implementation science and health equity. Pilot Studies. Our pilot data across 50 LGBTQ community centers shows that (1) training can improve providers' LGBTQ- affirmative CBT skills, (2) directors of LGBTQ centers unanimously support this training, (3) providers perceive that ongoing supervision can help maintain implementation but that receiving learning materials is also invaluable. Still, knowledge gaps remain for implementation science, including: (1) whether train-the-trainer strategies can further sustain implementation, (2) whether features of low-resource settings (e.g., high staff turnover) predict optimal implementation strategies, and (3) the target mechanisms through which EBP training impacts implementation and client outcomes. The proposed research will answer these questions. Method. Drawing on our long-standing partnership with the US's coordinating hub of LGBTQ community centers, in this type 3 hybrid trial we will randomize 90 centers to receive one of three strategies for implementing LGBTQ- affirmative CBT to compare their effectiveness (Aim 1), identify center-level moderators of implementation success (Aim 2), and examine the impact of the three strategies on client mental health through theory- informed organizational and provider mechanisms (Aim 3). Drawing on social learning theory, mental health research in low-resource settings, and our pilot data, we will compare three implementation strategies: (1) a suite of digital learning materials (materials only condition); (2) these materials plus weekly webinar training for 12 weeks (direct training condition); or (3) the above plus 1-year of supervision from a local supervisor who will receive expert consultation in a train-the-trainer format (local supervision condition). Implementation outcomes will include objectively coded provider fidelity in LGBTQ-affirmative CBT and mixed methods assessments of its reach, adoption, and maintenance across centers. Effectiveness outcomes will include client depression symptoms. Hypotheses. We hypothesize that the three additive training strategies will predict respective graduated increases in implementation success up to 2 years post-training. We hypothesize center-level determinants of implementation success from the Consolidated Framework for Implementation Research. We hypothesize organizational and provider factors that will mediate the stronger i...