Eastern Europe and Central Asia (EECA) have among the highest (5 of the top 10) incarceration rates worldwide, with staggering numbers of people who inject drugs (PWID), mostly with opioid use disorder (OUD: 82%) and HIV (PWH: 18%). The HIV epidemic in EECA is primarily concentrated in PWID, who account for 56% of new HIV infections with a high HIV prevalence among PWID (7.3-53.4%). The prisons and probation settings provide an opportunity for HIV prevention using opioid agonist therapies (OAT). The newly (past 5 years) formed probation system in these countries presents new opportunities and challenges for HIV prevention as probation does not have an international mandate to ensure human rights access to HIV prevention and treatment. This proposal builds on our existing implementation science network in several EECA countries. Specifically, first, MAT-LINK used SBIRT (screening, brief intervention, and referral to treatment) with trained researchers to pilot test a strategy to scale-up opioid agonist therapies. Second, many EECA countries have recently introduced probation, providing opportunities to ensure access to trans-institutionalized persons in probation to better align public health with public safety. Third, real-world implementation using existing infrastructure and staffing across more sites is now needed to expand OAT throughout prisons and probation. Fourth, we developed the first Project ECHO in EECA to guide OAT integration into primary care and psychiatric care into OAT settings. We have successfully used the EPIS (Explore, Prepare, Implement and Sustain) framework combined with the NIATx model of process improvement to scale-up OAT. Also using EPIS, we used Project ECHO to integrate OAT and HIV care into primary care clinics in Ukraine to teach specialized care to non-specialists, which we propose to use to teach prison doctors. NIATx is an evidence-informed facilitation strategy with documented successes in the adoption and scale-up of evidence-