The structure and function of the thyroid gland are described. A detailed analysis of population kinetics in the gland suggests a method of measuring cell survival after irradiation that has many features in common with methods used in other mammalian cell systems. This method is used to obtain survival curves for thyroid cells afer irradiation. The effects on survival of splitting the radiation dose into two or multiple fractions, radiation type, and radioprotective agents are also examined. In the light of these data the tolerance of thyroid tissue to radiation exposure under various conditions is discussed. The dosimetry and biological effects of 125I and 131I are described in detail, and compared with X-rays. Radioiodine treatment of thyrotoxicosis is presented in relation to the known biological effects of the isotopes on the gland. Carcinogenic action of ionizing radiations in the thyroid are reviewed with particular reference to the clinical consequences of observations in this field.