Physical and biologic principles, availability, and experience in use determine which radiopharmaceuticals will be the optimal agents to portray scintigraphically thyroid cancers. However, clinical reasoning must dictate which scanning procedure to use and when. To identify functioning thyroid nodules, and thereby exclude malignancy and also disclose the source of excess thyroid hormone, technetium Tc 99m pertechnetate is the agent of choice. To evaluate patients with metastatic differentiated thyroid cancer (DTC) for possible radiopharmaceutical therapy, low doses (no more than and probably less than 74 MBq or 2 mCi) of 131I iodide are preferred. 131I is especially necessary if dosimetry is undertaken to help prescribe the optimal therapeutic dose of 131I. For less well differentiated tumors that do not concentrate 131I, most experience has been with thallium 201, which enables depiction of most metastases, but, in time, newer agents may be found to produce superior scans. The goal of imaging of the less well differentiated cancers must be to direct additional therapy such as external beam radiation of surgical excision. It is unlikely that any scanning procedure that depends on the physiologic and biochemical activities of DTC will have as much sensitivity for locating tumors in the euthyroid as in the hypothyroid patient. A majority of medullary carcinomas of the thyroid (MCT) have been portrayed by indium 111 octreotide and by 99mTc pentavalent dimercaptosuccinic acid; other agents have concentrated to lesser levels in the tumors or have not been investigated as extensively. As of now, metastatic MCT defies therapy, and, therefore, scanning to detect the tumors has little clinical utility.