PROJECT SUMMARY/ABSTRACT The recently funded NIDA R24 entitled, Advancing The Science on Recovery Community Centers (RCCs) to Support Persons Treated With Medications for Opioid Use Disorder (R24 DA05198801) is an innovative project with the goal of cultivating and developing RCC research. Not included in this project, however, is a novel class of RCCs that support opioid and other substance use disorder recovery through peer-led physical activity and exercise. The goal of this supplement to R24 DA05198801 is to engage with and study a prototypical physical activity/exercise-based RCC known as The Phoenix, which provides a safe, recovery-oriented environment and a physically active community designed to attract and support individuals seeking addiction recovery. The Phoenix offers activities like CrossFit, rock-climbing, and yoga through a network of recovery-focused gyms. It has no fees, and its activities are run by peer volunteers who are themselves in addiction recovery. Its novel focus on physical activity, recreation, and integrating the biological with the psychosocial through leveraging the known positive benefits of engaging with others who have lived experience of addiction and recovery, has produced excitement in the field and garnered widespread interest from federal/state-level institutions, treatment systems, and criminal justice. Since its establishment in 2006, The Phoenix has grown rapidly to operate programming in 26 states with ~40,000 active members, and ~8,000 new members joining each year. The Phoenix has added programming in 25 new cities in 2020. Given the growing interest surrounding The Phoenix and its rapid expansion, there is a pressing need to understand its clinical and public health potential and explore moderators and mechanisms of its effects in a more systematic way. This need will be addressed in this study through the following aims that fit within the scope of the parent R24: 1) Engage Phoenix leaders in research- advancing activities promoting program evaluation and dissemination, and conduct focus groups with Phoenix members, peer instructors, and leadership to obtain greater contextual knowledge of participants’ experience with The Phoenix. 2) Conduct a comparative prospective, proof of concept, study (N=150; assessments at baseline, 1-, 3-, and 6-months) to test for improvements in substance use, psychosocial and psychophysiological functioning, relative to matched community controls, with psychophysiological assessments under resting and stress-reactivity conditions to explore changes in autonomic nervous system functioning. 3) Explore moderators of Phoenix’s effects that predict engagement, retention, and any derived benefits, as well as mediators through which Phoenix may confer benefit. Ultimately, this project will inform clinical decisions about patient referral to The Phoenix, and should Phoenix show signs of conferring benefit, create a foundation for future R01 clinical linkage trials.