Objectives: To develop a work-related medical rehabilitation (WMR) program for cancer patients based on the best available evidence, the expertise of rehabilitation professionals and the perspective of the patients, to ensure the fidelity of its implementation and to prepare its subsequent outcome evaluation. Material and Methods: The implementation study was based on organizational ethnography and action research, and followed a multimethod, participatory and iterative approach to data collection and analysis. The authors carried out observations in 4 rehabilitation centers and conducted focus groups with rehabilitation professionals and patients. The obtained data were subjected to qualitative content analysis. All findings were discussed promptly with the rehabilitation centers at feedback meetings that contributed to the further development of the program. Results: The following WMR modules were defined based on the findings: additional work-related diagnostics, multi-professional team meetings, an introductory session, work-related functional capacity training, work-related psychological groups and intensified social counseling. Process descriptions for the subsequent evaluation of the program via a cluster-randomized trial were also developed, containing, e.g., instructions for patient information and recruitment. Conclusions: Implementation studies can help to prepare for valid trials as they facilitate ensuring the feasibility, acceptability and fidelity of program implementation and evaluation. Organizational ethnography and action research are suitable methods for carrying out such studies. Int J Occup Med Environ Health. 2019;32(2):217–28