Background/rationale or Objectives/purpose Physical activity (PA) ameliorates many consequences of cancer treatment (e.g. fatigue, quality-of-life). We used mixed-methods with the aim of developing an intervention using a smartphone app to promote PA in breast, prostate and colorectal cancer survivors in the UK. Methodology or Methods A systematic review and meta-analysis synthesised available evidence for the effect of digital interventions on PA in cancer survivors (Study 1). Study 2 assessed interest in digital interventions in 5,840 cancer survivors and explored associated participant characteristics. Qualitative interviews sought cancer survivors’ experiences of using selected publicly available PA apps (Study 3) and cancer Clinical Nurse Specialists (CNSs) perspectives on PA promotion and the role of PA app interventions in cancer care (Study 4). Impact on practice or Results Study 1 showed that on average, digital interventions increase cancer survivors’ moderate-vigorous PA by 40 minutes per week. Study 2 found that 24% of cancer survivors report interest in app-based interventions. Interest was related to several sociodemographic and participant characteristics. Study 3 found that PA apps must acknowledge the varying needs and preferences of cancer survivors. Apps that promote walking were favoured and recommendations from CNSs are valued. Study 4 showed that CNSs were positive about the use of apps to complement existing PA promotion in cancer care and provided insight into how app-based PA interventions could be implemented in routine cancer care. Discussion or Conclusions Together, these studies led to the development of a randomised controlled trial using a PA app in cancer survivors, which has secured funding and will be the next phase of this research.