With increasing numbers of people being diagnosed with cancer, and living for many years after diagnosis, it is important that cancer survivors are supported to live well, for as long as possible. Physical activity improves quality of life and many treatment side effects after cancer. Physical activity is important in reducing the risk of other common comorbid conditions and may prolong survival in cancer survivors. However, many cancer survivors do not meet recommended levels of physical activity and evidence-based interventions that are accessible and that could be implemented at scale are needed. The rising use of the internet, smartphones and mobile technology mean that digital interventions could reach large proportions of the population in a way that could be more cost-effective and scalable. This thesis used mixed-methods to conduct a series of studies with the aim of developing a smartphone-app based intervention to promote physical activity in breast, prostate and colorectal cancer survivors. Study 1 showed that digital interventions have the potential to increase cancer survivors' moderate-vigorous physical activity by 41 minutes per week, however high-quality studies are lacking. Study 2 found that 38% and 24% of breast, prostate and colorectal cancer survivors respectively are interested in internet-based and app-based health behaviour interventions. Interest is related to several sociodemographic and participant characteristics. Study 3 found that physical activity apps must acknowledge the varying needs and physical activity preferences of cancer survivors. Apps that promote walking and are recommended to them by members of their clinical team (particularly Clinical Nurse Specialists) are favoured. Study 4 provided insight into Clinical Nurse Specialists' perceptions of their role in physical activity promotion and showed that they are generally positive about the use of apps to complement existing physical activity promotion in cancer care. Together, these studies led to the development of an app-based physical activity intervention for cancer survivors.