Background: To improve the quality and efficiency of prostate cancer survivorship care, a randomized controlled trial (RCT) is currently comparing the safety and effectiveness of specialist- (usual care) versus primary care-based (intervention) prostate cancer follow-up. This process evaluation assessed the reach and identified constructs for the implementation of primary care-based follow-up in a RCT setting. Methods: A mixed-methods approach was used through the Reach, Effectiveness, Adoption, Implementation, and Maintenance (RE-AIM) framework and the Consolidated Framework for Implementation Research (CFIR). We used quantitative data to evaluate the reach of the RCT and qualitative data (interviews) to indicate the perspectives of patients, general practitioners (GPs) and specialists. Thematic analysis was used to analyze the interview transcripts. Results: In total, 569 patients with localized prostate cancer from 12 hospitals were invited to participate in the trial. 18 patients were not eligible, 145 patients declined (of whom most preferred follow-up in the hospital), whereas 21 GPs declined to participate. Finally, 385 patients were randomized to specialist- (n=192) or to primary care-based (n=193) follow-up. In addition, we interviewed 15 patients, 10 GPs and 8 specialists. Participants identified several advantages of primary care- versus specialist-based follow-up: it is closer to home, more accessible, the relationship is more personal, and the hospital can focus on patients undergoing active treatment. Nevertheless, participants also identified challenges: evidence-based guidelines should be implemented, communication and collaboration between primary and secondary care should be accessible and transparent, quality indicators (i.e. PSA levels) should be collected, and GPs expect compensation (money or extra capacity). Conclusions: If the RCT shows that primary care- is equally effective as specialist-based follow-up, this study could enable the transition of prostate cancer follow-up to primary care by presenting information on the reach of an RCT and by providing advantages and challenges of primary care-based prostate cancer follow-up.