Aims:The overall aim of this thesis was to increase knowledge about the promotion of physical activity (PA) in patients during and after curative cancer treatment. Methods: Study I investigated agreement between a 7-day diary and the SenseWear Armband mini (SWA) when assessing PA and sedentary time in 65 participants with breast cancer. Agreement was examined with Bland-Altman plots. Studies II-IV used data from an RCT, where participants about to start curative treatment for breast, prostate or colorectal cancer, were randomised to six months of high or low-to-moderate intensity exercise (supervised group-based resistance- and home-based endurance training), with or without self-regulatory behaviour change techniques (BCTs; e.g. goal-setting and action planning). In study II, participants randomised to the groups with self-regulatory BCTs answered a questionnaire (n=229) and took part in semi-structured interviews (n=18) after exercise programme completion. The interviews were analysed with thematic analysis. In Study III, all participants (n=577) randomised were included. Exercise adherence during the intervention was assessed with attendance records, training logs and heart rate monitors. Regression analyses were performed to determine the effect of self-regulatory BCTs on exercise adherence. In Study IV, participants (n=301) who provided data about their PA level at 12-month follow-up were included. Regression analyses were performed to determine the effect of self-regulatory BCTs on PA maintenance and potential baseline predictors. Results: There were mean differences and wide limits of agreement between the 7-day diary and the SWA when assessing PA and sedentary time. Participants described different incentives to exercise, which fostered feelings of autonomy, competence and relatedness and helped them find motivation to exercise during cancer treatment. Social support from coaches, feedback, self-registration of exercise and scheduled sessions at a public gym were useful for exercising. There was no effect of the self-regulatory BCTs on exercise adherence during the intervention; however, a positive effect was found on PA maintenance at 12-month follow-up. Baseline predictors of PA maintenance at 12-month follow-up were health-related quality of life, exercise motivation, expectations of exercise and being a former or current smoker/snus user. Conclusions: A 7-day diary and the SWA have limited agreement and cannot be used interchangeably in patients with breast cancer. Individual incentives and a positive and supportive environment are crucial to increase exercise motivation in patients undergoing curative cancer treatment, and can be promoted using specific support (i.e. social support, feedback, self-registration of exercise and scheduled exercise sessions). The addition of self-regulatory BCTs is not likely to improve exercise adherence in patients undergoing curative treatment for breast, prostate or colorectal cancer and participating in structured, well-controlled exercise interventions, but may improve long-term PA maintenance in this population. Patients with low health-related quality of life, low exercise motivation, high expectations of exercise or with a history of tobacco use at the start of their cancer treatment may be those most in need of such support to maintain PA.