Recent advances in the diagnosis and treatment of cancer have produced significant improvements in survival rates for many cancer types. The sequential combinations of treatment modalities (chemotherapy, radiotherapy, immunotherapy and hormonal therapy) aim to cure or prolong life. However, they are also related to debilitating side effects that negatively impact upon quality of life. Consequently, research and interventions that target these treatment related toxicities will have considerable benefit in this population. In this thesis, the results from three studies are presented. The first study [chapter two] is experimental in nature and investigates the mechanisms of physical fatigue in patients with breast cancer receiving adjuvant chemotherapy. The study identifies the importance of investigating a subtype of fatigue, operationally defined as an increase in perceivedexertion during a physical task. As hypothesised, there was an increase in perceived exertion at baseline and post chemotherapy in these patients, and the study had the capability to relate this increased perception of effort to different physiological and psychological processes associated with cancer therapy. The second randomised controlled experiment [chapter three] indentifies a safe and effective exercise intervention appropriate for reversing physiological and psychological side effects associated with cancer therapy in patients with prostate cancer. As hypothesised, a high intensity progressive resistance training programme improves body composition, physical function, fatigue and quality of life. Interestingly, this study identifies improved mental aspects, as well as physical aspects of fatigue in response to an exercise training programme. The third cross sectional study [chapter four] identifies treatment related barriers to exercise in patients on follow-up from breast cancer treatment. This study identified treatment related side effects, such as fatigue and increased perception of effort to be the most common reasons for not engaging in physical activity after treatment for breast cancer. This study provides novel targets for future research and treatment of fatigue with this patient population. In conclusion, this thesis presents mechanisms by which fatigue is increased in breast cancer patients during adjuvant chemotherapy; identifies potential barriers to physical activity in breast cancer patients on follow-up from treatment; and also adds to existing literature that exercise can alleviate the side effects of treatment from cancer. Overall, the studies in this thesis provide a better understanding of the increased perception of effort associated with fatigue in patients treated for cancer and provides novel targets for intervention strategies.