Living beings have alwavs been exposed to ionizing radiation from various natural sources. Nevertheless, the discovery of X-rays by Roentgen in 1895, and of radioactivity in uranium salts by Becquerel in 1896, brought, in addition to very great benefits, unforeseen hazards. Considerable damage resulted until the first measures of precaution were adopted. Indeed, within only five years, 170 cases of radiation injury were recorded.