Background: After a cancer diagnosis, physical activity and healthy eating can help cancer survivors to maintain better physical and mental health. This study compared two models to help understand factors that predict physical activity and nutrition in survivors of breast or prostate cancer. The transtheoretical model (TTM) focuses on stage of readiness to engage in a behaviour, and the commonsense illness representations (CM) model looks at individual cognitive and emotional perceptions of the illness. Methods: Participants were 92 men who had been diagnosed with prostate cancer and 154 women who had been diagnosed with breast cancer. Participants completed a written or online questionnaire which assessed demographic and health information, illness representations, diet and exercise stage of change, self efficacy and preferences regarding health behaviour interventions. Health behaviours in the past 7 days were measured via the International Physical Activity Questionnaire and concordance with national age- and gender-specific dietary guidelines. Results: The TTM model was related to physical activity in both male prostate cancer survivors and female breast cancer survivors. Exercise stage of change and exercise self-efficacy were significant independent predictors of activity in multiple regressions for both survivor groups. This model was also supported for women's nutrition scores but not for men's. The CM model was not supported in this study. Cancer survivors more frequently reported increasing physical activity and healthy eating since diagnosis than maintaining or decreasing these behaviours and many expressed interest in interventions that would assist with activity and healthy eating. Conclusion: Health behaviours among survivors of breast cancer or prostate cancer can be partially explained by the TTM model of behaviour change. Further investigation of the commonsense illness representations model in cancer survivors may require refinement of methods, as items referring to "your illness" appeared to be viewed as having limited relevance for many survivors. Research Implications: The applicability of the TTM model to health behaviours of cancer survivors could be further investigated through longitudinal research, intervention research, and studies with more targeted groups of survivors. Clinical Implications: Based on these results, it is likely that interventions based on the TTM model of behaviour change would be helpful for cancer survivors who want assistance with increasing their physical activity and/or healthy eating. This study showed that many survivors take their own steps to improve these health behaviours, and thus a stepped-care approach may be appropriate, with higher intensity health behaviour support available to those who require this type of support. Acknowledgement of Funding: None.