An overview is given of the research carried out by the French Atomic Energy Commission on the geological disposal of nuclear waste. Recalling the new legal framework of these studies, the specific methodology of this field, which relies on the association of laboratory and in situ experiments, natural analogues, and modeling is emphasized. Next are underlined the basic questions, which have to be reliably answered when one wants to predict the behavior of the repository and the fate of the most radiotoxic nuclides, in particular over very long time spans: What parameters govern the hydrodynamics and the geochemistry of the system? What are the processes controlling the degradation of the different technological and natural barriers, as well as the release, migration, and retardation of radionuclides through the geosphere? For each of these issues, the current programs of research are described and indications are given about significant achievements.