Many cancer survivors who finish treatment experience long-term consequences that can reduce their quality of life (QoL). Healthy behaviours (e.g. physical activity) can improve cancer survivorsā€™ QoL. A digital intervention, titled ā€œRenewedā€, was developed to improve the QoL of breast, colon and prostate cancer survivors in primary care by providing support about physical activity, healthy eating, weight management and psychological well-being. For some, Renewed also included the option of brief support from a healthcare professional. A separate three-arm randomised controlled trial (Renewed, Renewed with support or control conditions) showed that those in the supported arm significantly increased their QoL at 12 months. Furthermore, prostate cancer survivors in the supported arm had better improvements in QoL compared to other cancer survivors. To enhance understanding of the trial results, this thesis aimed to conduct a process evaluation of Renewed to understand how the intervention achieved change in QoL, for whom, and under what circumstances. The overarching aims of this thesis were to explore, describe and characterise the mechanisms through which Renewed achieves change in QoL in cancer survivors and the barriers and facilitators to implementation by cancer survivors and healthcare professionals. Three papers explored these aims. Firstly, a qualitative study exploring thirty-three cancer survivorsā€™ experiences using Renewed through interviews suggested that the majority found the intervention easy and convenient, with some reporting perceived behaviour changes with minimal use. However, some individuals were less motivated to use Renewed when they had comorbidities or if they joined the study to ā€˜give backā€™ or contribute to research. Additionally, prostate cancer survivors reported less availability of support outside of the intervention compared to breast and colon cancer survivors. Secondly, a quantitative usage analysis examined the relationships between using Renewed, accessing support, QoL and individual characteristics. This study demonstrated that while 55.1% (n=970/1760) of cancer survivors did not access the optional content, using Renewed more was related to better improvements in QoL. Finally, a qualitative interview study explored twenty-eight healthcare professionalsā€™ experiences supporting those using Renewed. This study found that an approach where the expertise is provided by the intervention and brief additional support provided by a healthcare professional is an acceptable way to overcome key barriers to supporting cancer survivors in primary care. Additionally, whilst most HCPs cope well with a non-directive approach, a minority may require more support to feel confident implementing this. Together, these findings suggest a digital intervention like Renewed may be suitable for implementation in primary care with minor changes. Implications suggest that accessing support may be important to motivate greater engagement but not a critical factor in improving QoL and that presenting novel information earlier in an intervention may help to motivate continued engagement.