||ackground: Pilot Oncogeriatric Coordination Units (UPCOGs) were created by the French National Cancer Institute (INCA) in order to implement routine geriatric assessment of all cancer patients over 75 years of age. This article examines the role of geriatric and oncologic tools in the organization of medical oncogeriatric activities, focusing on the role and place of geriatricians. Methods: We conducted a qualitative sociological survey in the West Paris Oncogeriatric Program (POGOP), one of the Pilot Oncogeriatric Coordination Units (UPCOGs) recently created in France. Various qualitative methods were used including a review of the literature, participative observational surveys, and semidirective interviews with medical staff managing elderly cancer patients. Results: The results show that the way in which geriatric assessment procedures are implemented confirms the role of the geriatrician in the diagnosis and prevention of vulnerabilities and fragility at the time of initial diagnosis and medical decision making. Nevertheless, the articulation of these different working methods gives rise to various organizational configurations. Conclusions: The POGOP has largely contributed to clarifying medical activity in oncogeriatrics: identification of physicians, definition of shared goals, initiation, and structuring of new partnerships. Nevertheless, the geriatrician's tools, expertise, and know-how are often perceived ambiguously.