Physical activity (PA) participation has been shown to be helpful in improving physical and mental well-being among cancer survivors. The purpose of this chapter is to review the literature on the determinants of physical activity motivation and behavior among cancer survivors. Using theories of behavior change, researchers have sought to identify the correlates of motivation that predict the participation in regular physical activity in observational studies, while intervention studies have focused on manipulating those factors to support the initiation of physical activity. The majority of this work has been conducted with breast cancer survivors, and there is an interest in expanding this work to survivors of others cancers (e.g., prostate, lung, and colorectal cancer). Results suggest that constructs from the Theory of Planned Behavior (TPB), Transtheoretical Model (TTM), and Social Cognitive Theory (SCT) are associated with greater motivation for physical activity, and some of these constructs have been used in interventions to promote physical activity adoption. There is scope for understanding the determinants of physical activity adoption in various cancer survivor populations. Much more needs to done to identify the determinants of maintenance of physical activity.