The common sources of ionizing radiation exposure to the thyroid gland in humans are accidental environmental exposure and medical, therapeutic, or diagnostic irradiation. Radiation often induces notable histologic changes in the thyroid tissue and is a well-established risk factor for benign and malignant thyroid tumors. In this paper, we review the acute and chronic histologic changes in the thyroid gland subjected to irradiation, and characterize benign thyroid nodules and malignant tumors arising after exposure, with particular emphasis on thyroid lesions in the population exposed to ionizing radiation as a result of the Chernobyl nuclear accident.