Background Adolescent and young adult survivors of childhood cancer (AYA) are at risk for treatment-related late effects (eg, heart and lung problems) which may be mitigated by physical activity (PA). To design effective, tailored PA interventions for this population, predictors and benefits of PA behavior need to be measured in real-time.Purpose To examine the feasibility and acceptability of ecological momentary assessment (EMA) combined with accelerometry and explore the dynamic associations between PA and real-time physical and psychosocial factors among AYA.Methods AYA (N = 20, mean age = 18.9 years) recently off cancer treatment participated in a 2-week intensive monitoring protocol in which they completed up to 4 EMA surveys/day assessing current mood, pain, fatigue, arousal, PA intentions and motivation, and social-environmental context, while PA levels were passively monitored using a wrist-worn ActiGraph GT9X accelerometer. Acceptability was measured via self-report.Results EMA and accelerometry were feasible and acceptable (>= 70% compliance and study endorsement) for AYA. Multilevel models showed that AYA engaged in more PA when they were away from home, with others, in a better mood, less fatigued, more energetic, and more motivated than their own average levels. Further, when AYA engaged in more PA than their usual levels in the hour before completing an EMA survey, they subsequently reported less fatigue, less pain, more energy, and a more positive mood.Conclusions EMA and accelerometry are acceptable and feasible among AYA survivors of childhood cancer. This methodology can be utilized for understanding the real-time barriers, facilitators, and benefits of PA behaviors in this at-risk population to design effective, dynamic PA interventions. It is important for adolescent and young adult survivors of childhood cancer to engage in physical activity to improve their long-term health. However, many things can get in the way of being active during the day, and physical activity may have other daily, real-time benefits. This study explored whether answering daily surveys through a smartphone app combined with activity tracking is a useful and acceptable way to gather more insights about physical activity behavior. Over 2 weeks, 20 participants (aged 15-29) answered short surveys (4/day) about their mood, energy, fatigue, pain, and social setting while their activity levels were measured with a wrist-worn activity monitor. The results showed that participants were satisfied with the study and completed most surveys and wore the monitor most days. Participants were more active when they felt more motivated, energized, in a better mood, or with others. Doing more activity than usual improved their mood, energy, and reduced fatigue and pain shortly after. This method provides a deeper understanding of the daily factors influencing physical activity for young cancer survivors and can help us create more personalized interventions to help them stay active and healthy.