Up to now, radiation oncologists had at their disposal only a number of well-known histological and clinical factors in order to define the optimal dose which should be delivered to a given tumor. Recently, radibiological studies have suggested additional parameters which may play a major role in tumor radiocurability. These parameters are: the number of clonogenic cells, intrinsic radiosensitivity, hypoxia and proliferation kinetics. "Predictive" tests are being developed and evaluated for each of these parameters. The more advanced studies deal with intrinsic radiosensitivity; preliminary data show impressive variations in radiosensitivity within groups of clinically "homogeneous" tumors. Should these tests prove to be reliably predictive of radiocurability, it will be possible, in the near future, to propose to any given patient a "custom-made radiotherapy" adapted to the precise features of his or her tumor.