Survivors of adolescent and young adult (AYA) cancer endure a range of symptoms and side effects, including weight gain, unfavourable changes in body composition, physical disfigurements, and tissue damage, which can impair their physical and psychological health. Identifying and optimizing interventions that can mitigate negative side effects are necessary. Whereas physical activity has been identified as one such intervention for child and older adult cancer survivors, the evidence for survivors of AYA cancer is far less convincing to put forward recommendations and argue for integrating physical activity into practice. Research seeking to understand if/how and under what circumstances physical activity is related to physical and psychological outcomes is of particular importance to guide care aimed at relieving cancer-related damage and distress. This research program sought to address this through three interrelated studies, which progressed sequentially. Study One examined the clarity, appropriateness, and relevancy of eight widely used questionnaires assessing self-reported physical activity and psychological outcomes. Cognitive interviews were conducted with seven survivors of AYA cancer who were representative of individuals likely to participate in future research. Findings from this study indicated most of the questionnaires tested could be used to assess self-reported physical activity and psychological outcomes among survivors of AYA cancer with or without slight modifications. After incorporating participants' feedback into the questionnaires, Study Two was conducted to: (1) examine the cross-sectional relationships between physical self-perceptions and self-esteem (physical and global), and (2) explore if physical activity and/or self-efficacy for physical activity moderated these relationships. Correlation analyses were conducted, and as hypothesized, physical self-perceptions and physical and global self-esteem were positively related. Hierarchical multivariate linear regression analyses showed that self-efficacy for physical activity moderated the relationship between physical self-perceptions and physical self-esteem (but not global self-esteem), such that at higher levels of self-efficacy for physical activity, the magnitude of the association between physical self-perceptions and physical self-esteem was greater. Physical activity did not moderate the relationship between physical self-perceptions and physical or global self-esteem. These findings provide empirical evidence that physical self-perceptions and self-esteem (physical and global) are related and suggest self-efficacy for physical activity may serve to strengthen the relationship between physical self-perceptions and physical self-esteem among survivors of AYA cancer. However, more work exploring if/how and under what circumstances physical activity is implicated in this relationship is needed via experimental study designs. In Study Three, a two-arm, mixed-methods randomized controlled trial (RCT) designed to test a 12-week physical activity intervention among survivors of AYA cancer was piloted. The objectives of this study were to assess the feasibility and acceptability of trial methods and the intervention. Findings suggest modifications to the methods and intervention are required. Making the trial multi-site, using multiple recruitment strategies, refining assessments of directly-measured physical activity behaviour and aerobic capacity, and incorporating behavioural support into the intervention may improve feasibility and acceptability. This study highlights the value of pilot trials and provides useful data that can be used to optimize trial methods and physical activity interventions for this population. Collectively, the studies comprising this thesis lay the foundation for further testing of if/how and under what circumstances physical activity and physical and psychological outcomes are related in survivors of AYA cancer. Findings offer researchers and practitioners information to aid in the development of theoretically- and empirically-based interventions seeking to mitigate the adverse effects of cancer and its treatments in this population.