Epidemiological studies have shown an increased incidence of thyroid tumors after external irradiation during childhood. These studies have shown a long latency and have pointed out risk factors, related either to the subject (young age, female sex) or to the radiation (dose, fractionation, protaction). Irradiation by Iodine 131 in adulthood does not seem to increase the risk of thyroid tumor but in childhood data are too scarce to allow any conclusion. Similarly, the risks following low radiation doses and/or protacted radiation exposure are still matter of debate.