This text is intended to introduce the following articles, which correspond to the lectures given at the "Radiosensitivity" session of the 1998 SFRO Meeting. We first underline the somewhat disappointing results obtained so far for evaluating tumoral radiosensitivity. However, a few new tests could bring some hope in a not-too distant future. We then focus on individual radiosensitivity. We first list the syndromes or diseases associated with a known hyperradiosensitivity. We then concentrate on the radiosensitivity tests that are available in 1998, some of them being already used in a few centers in specific situations. However, the "optimal" test is still to be identified. After a brief chapter on the possible modulations of radiosensitivity, we conclude with a few recommendations to the clinician.