There are over half a million young adult cancer survivors (YACS) in the United States. YACS are an underserved and vulnerable subgroup of survivors that experience increased risk over time for obesity and chronic diseases such as diabetes and cardiovascular disease. Increasing physical activity (PA) is a promising behavioral intervention that has positive effects on physical function, body composition, cardiovascular fitness, and health-related quality of life. To date, few interventions have been designed specifically to promote PA in YACS, and none have been successful in promoting long-term adherence to PA guidelines of 150 minutes/week of moderate-to-vigorous intensity activity. Effective PA interventions have used self-monitoring as a behavior change technique to help individuals monitor daily activity, set goals, and enhance motivation. In our pilot randomized trial of a social cognitive theory-based, Facebook-delivered PA intervention for YACS (n=86), intervention participants increased self-reported total PA compared with a self-help group (237 vs. 76 minutes/week; p=0.07, d = 0.39), which was driven by differences in light PA (164 minutes/week vs. 29 minutes/week; p=0.03). The emergence of lower-cost and widely available wearable activity trackers represents a unique opportunity to simplify self-monitoring and deliver more precisely tailored PA interventions that dynamically adapt goals and messages in response to an individual's changing activity patterns over time so as to provide more relevant and timely support. Building on the promise shown by our pilot study and utilizing self-regulation approaches that our team has used to target PA as part of a successful weight gain prevention intervention in young adults, we aim to bolster intervention effects on PA using activity trackers and theory-based enhancements. This research proposes a randomized controlled trial to test the efficacy of a theory-based, mobile PA intervention with adaptive goal-setting and tailored feedback that is aimed at increasing PA among YACS. YACS (n=280), diagnosed between ages 18-39, will be recruited and randomized into one of two conditions: 1) activity tracker + Facebook group (Self-help) or 2) activity tracker + Facebook group + adaptive goal-setting and tailored feedback (Intervention). We plan to enroll YACS in a 6-month intervention followed by an additional 6 months of tapered contacts to address the following specific aims: 1) Determine the effects of a theory-based, mobile PA intervention compared with a self-help control group on changes in total PA. 2) Determine whether changes in social cognitive factors mediated the intervention effects on PA outcomes. 3) Explore whether intervention effects differed across potential demographic and healthrelated moderators. Assessments of objectively-measured PA (using ActiGraph accelerometers) and other outcomes will be completed at baseline, 3, 6, and 12 months. In the proposed study, we seek to promote PA among an underserved population of YACS using a high-reach, low-cost, technology-based strategy that can be adopted on a larger scale and thus has high potential for reducing cancer-related morbidity and disparities.