The available information on the induction of thyroid cancer in humans by ionizing radiation is summarized and weaknesses or gaps in assessing risk are identified. Issues to be addressed include: average estimates of thyroid cancer risk from external irradiation, the effects of age on thyroid cancer induction, shape of the dose-response curve for acute irradiation, magnitude of risk at low doses, effects of dose fractionation or dose protraction, the relative effectiveness of iodine-131 (131I) in inducing thyroid cancer compared to external radiation, the temporal course of radiogenic thyroid cancer risk, mortality caused by thyroid cancer, host-susceptibility factors for radiogenic thyroid cancer, and biological factors in risk. It is concluded that the most important needs are to obtain more information on thyroid cancer risks following low-level or highly fractionated radiation exposures and following 131I exposure in children.