This article reviews the technical aspects and clinical applications of the radioactive iodine uptake test and thyroid scintiscanning. The choice of radionuclide for the tests is discussed. The main uses of the radioactive iodine uptake test are to identify the cause of hyperthyroidism and to aid in the selection of the I-131 dose in the treatment of hyperthyroidism. Factors other than thyroid diseases that alter uptake results are identified. Thyroid scintiscanning is used in the identification of normal and ectopic thyroid tissue, in the diagnosis of the cause of a patient's hyperthyroidism, and as part of the evaluation of selected patients with thyroid nodules.