Returning to and maintaining employment after cancer is essential for restoring social participation, financial independence and reducing the social costs associated with cancer. Many obstacles that prevent or delay the return to work have been identified. They are associated with the consequences of the disease and treatments such as fatigue, pain and cognitive disorders, the lack of collaboration between health professionals (oncologists, general practitioners and occupational physicians), and the characteristics of the environment. in terms of the demands of the job, and the (lack of) social support from superiors and colleagues. There are social inequalities in the return to work after cancer, with a poorer professional prognosis among older and less qualified people. Social inequalities linked to ethnicity have been documented in other countries. In France, 58,500 new cases of breast cancer are diagnosed each year, half of them in women of working age. The importance of job retention has been formalized in the objectives of the latest cancer plans, and in the 2018-2022 national health strategy. Initiatives are observed to promote this issue by employers: development of a charter by the National Cancer Institute and in the associative field aimed at promoting support practices in their professional environment for people with cancer. Despite the development of descriptive knowledge on prognostic factors for returning to work after cancer, the results of interventional studies are mixed. No intervention has been shown to be effective in facilitating return to work and reducing social disparities in employment after breast cancer. Interventions have been criticized for being too medicalized and lacking a sufficient theoretical basis to analyse causes (theory of the problem) and propose solutions (theory of action). The FASTRACS intervention was developed with the Intervention Mapping protocol to facilitate the return to work after breast cancer, it defines a return-to-work path from the hospital to the company through care primaries. This intervention is anchored in primary care with an early transition consultation in general medicine in the month following the end of active treatments (chemotherapy or radiotherapy according to the care protocol). This positioning in primary care allows a bio-psycho-social assessment of the needs of women after cancer. This consultation was designed to establish a plan of care and return to work according to the temporality and individual needs of each woman. It will make it possible to determine the right time for the pre-recovery visit in order to anticipate the professional challenges of the recovery. Return to work/maintenance in employment interventions are complex social interventions, implemented by social actors who act in an environment with which they interact. These interventions (or programs) present an increased risk of failure in their implementation and sustainability. Realistic evaluation comes from the trend of theory-based evaluation (theory-based or theory-driven evaluation). According to this approach, social interventions are based on theories, which can be tested through empirical observation to better understand how and why they produce their effects, and in what context. This approach overcomes the limits of the black box model. It is recommended to inform the public decision to interrupt, modify or intensify an intervention. It aims to answer the question: what works, how, why, for whom, and under what circumstances? . It aims to describe the mechanisms of the effectiveness of an intervention by linking its effects to the characteristics of its implementation context (search for CME configurations = context - mechanisms - effects). The search for these configurations, otherwise called half-regularities because they can be observed empirically with certain variations, is intended to develop a middle-range theory with a sufficient level of generalization and abstraction. to explain the tendencies and the regularities observed in the interactions contexts-mechanisms-effects of the intervention. This approach is particularly indicated in the evaluation of the FASTRACS intervention, given the complexity of the intervention, the number of actors involved, and the variety of its implementation contexts.