This chapter discusses positron emission tomography, where varieties of radiotracers, labeled with positron-emitting radionuclides, have enabled assessment of a variety of metabolic and other functional parameters. The ability to image the photons emitted when a positron collides with an electron has revolutionized our ability to image nanomolar quantities of positron-labeled radioactive substances. Positron emission tomography utilizes these photons, which are emitted in a discrete energy at opposite directions from each other, to create a tomographic image of the distribution of the radioactivity. FDG PET has also been utilized to detect and stage thyroid carcinoma. The increasing use of FDG PET for staging and management of a variety of cancers has resulted in the serendipitous detection of hypermetabolic thyroid lesions in 4–14 percent of patients with other cancers and without a palpable thyroid nodule.