The role of PET in the diagnosis and management of thyroid cancer is discussed. The major role of F-18 FDG PET is in patients with discordant negative I-131 scan and a positive serum thyroglobulin values. F-18 FDG PET scan localized metastatic sites in I-131 scan-negative thyroid carcinoma patients with high accuracy. F-18 PET is also valuable in medullary thyroid cancer with high calcitonin level. Focal thyroid uptake in patients with non-thyroidal diseas has high likelihood of thyroid cancer. (Korean J Nucl Med 2002;36;147-54)