The avidity of differentiated thyroid carcinoma for iodine is the basis for the use of radioiodine (131I) both for the detection and the treatment of recurrent thyroid cancer in patients following initial surgical treatment (thyroidectomy). Because recurrence of this type of cancer may be delayed for many years, long-term follow-up is needed. Nowadays such surveillance involves clinical assessment, monitoring of serum thyroglubulin, and, when indicated, whole-body imaging with 131I. Sensitivity of 131I imaging depends on proper preparation of the patient and careful attention to imaging technique. Interpretation of images requires knowledge of physiologic sites of radioiodine concentration and causes of artifacts. Because of the potential suppressive effect of the radiation from a diagnostic administration of 131I on the uptake of a subsequent therapeutic administration (so-called stunning of thyroid tissue) many centers limit the amount given for scanning to 2-3 mCi (74-111 MBq). Several tumor-seeking radioisotopic agents other than radioiodine have shown promise for improving the detection of metastases, and some of these agents offer a useful adjunct to 131I in the management of selected patients, particularly in those with suspected metastatic disease and negative 131I scans.