Project Abstract Most breast cancer survivors do not engage in regular physical activity (PA). Our piloted PA behavior change intervention for breast cancer survivors significantly improved PA and health outcomes post-intervention. Testing in additional sites with longer follow-up is warranted to confirm program effectiveness short and longer term. Importantly, the pilot intervention resulted in changes in PA and social cognitive theory constructs, enhancing our potential for testing mechanisms mediating PA behavior change. Therefore, we propose a multi- center, randomized controlled trial enrolling 256 breast cancer survivors with the following study aims: Primary study aim: To examine intervention effectiveness, the primary study aim is to compare the effects of the BEAT Cancer PA behavior change intervention to usual care on short and longer term PA adherence among breast cancer survivors. We hypothesize that, compared with usual care, the intervention will result in a significant increase in PA adherence post-intervention that will be maintained 3 and 9 months post- intervention. Secondary study aim #1: To better understand the mechanisms of PA behavior change, secondary study aim #1 is to compare the effects of the BEAT Cancer PA behavior change intervention to usual care on social cognitive factors and examine if such changes mediate PA behavior change. We hypothesize that, compared with usual care, the intervention will result in significant improvements in social cognitive factors targeted by the intervention. We also hypothesize that social cognitive factors will mediate the intervention effect on PA with selected factors mediating the intervention effect on self-efficacy and the effect of self-efficacy on PA. Secondary study aim #2: Although the primary focus of this proposal is to achieve and understand PA behavior change, we also aim to compare the short and longer term health effects of the BEAT Cancer PA behavior change intervention when compared with usual care. We hypothesize that, compared with usual care, the intervention will result in significant improvements in fitness, muscle strength, waist-to-hip ratio, quality of life, fatigue, sleep quality, and joint dysfunction. Outcome measures obtained at baseline, 3 months (i.e., immediately post-intervention), 6 months, and 12 months will include accelerometer (PA and sleep), survey (social cognitive factors, quality of life, fatigue, perceived sleep, and joint dysfunction), submaximal treadmill test (fitness), dynamometer (muscle strength), and waist-to-hip ratio. The study aims will be analyzed with mixed-model analysis of variance (ANOVA) with the secondary study aim #1 also utilizing multivariate analysis of variance (MANOVA) and Freedman- Schatzkin difference-in-coefficients test of mediation. Identifying the effects and underlying mechanisms of PA behavior change interventions will facilitate translation to community settings for improving the health and well- being of breast cancer survivors.