Doctor of Philosophy (PhD) / Exercise and physical activity have many benefits for adults with cystic fibrosis (CF), including the potential to aid mucus clearance, improve lung function, exercise capacity and quality of life. Despite the recommendations from consensus documents for CF adults to engage in regular physical activity, exercise participation amongst this population is often very low. No in-depth study has been undertaken to explore the determinants of exercise participation for this group and no studies have examined the benefits of habitual physical activity on the health status and quality of life of CF adults. As well, the current methods to quantify physical activity are problematic. The series of studies, involving CF adults, in this thesis was therefore undertaken in order to examine the physiological rationale for the use of exercise as an airway clearance technique, to understand the decision making process to engage in exercise, to determine if health status and quality of life were affected by exercise participation, and to establish the accuracy of a recently-developed objective measure of physical activity. The study in Chapter 2 provided some physiological rationale for choosing treadmill exercise to aid airway clearance in CF. The main findings were that a single bout of moderate intensity exercise increased the subjective ease of expectoration, most likely due to the increased ventilation with exercise, and that sputum viscoelasticity was favourably decreased immediately following treadmill exercise compared to cycle exercise and control. The studies in Chapters 3 and 4 identified the main beliefs regarding exercise for CF adults and highlighted that the main predictors of exercise intention and participation for this group were aspects of perceived and actual control to exercise, namely self-efficacy or confidence to exercise, feeling healthy, receiving encouragement to exercise, and rating exercise as a high priority treatment. Positive ratings of these aspects of control either increased exercise participation directly, indirectly by increasing intention, or strengthened the conversion of exercise intention to participation. Strategies aimed at targeting these aspects of control are therefore likely to be effective in increasing exercise participation for CF adults. The study in Chapter 5 demonstrated that CF adults, who reportedly performed at least 90 minutes of moderate to strenuous exercise per week, had significantly higher quality of life and fewer days in hospital over the following year than their peers, who exercised less. The difference in hospitalisation between the CF adults, who reportedly exercised more than 90 minutes per week and those who did not, was independent of baseline lung function, and the females who reportedly performed less than 90 minutes of exercise per week had three times as many days in hospital than their high-activity peers. The study in Chapter 6 showed that the SenseWear Pro3 Armband activity monitor provided a reasonable estimate of energy expenditure and step count. Also, its accuracy was not affected by diagnosis with CF, despite the potential for the high salt content in the sweat to interfere with the device’s physiological sensors placed on the skin. Overall, this series of studies adds to the growing evidence of the physical and psychological benefits from exercise participation for CF adults, as well as providing some empirical evidence upon which to base strategies to improve exercise participation for this group and support for an objective measure of physical activity.