Several causative factors for thyroid cancer have been identified, the most important of which is low-dose ionizing radiation. Although the prognosis for patients with differentiated thyroid cancer is generally good, the literature continues to present new information concerning clinical, pathologic, and molecular factors that allow for identification of high-risk subgroups. Molecular biology techniques now enable clinicians to identify patients with benign disease or a family history of thyroid cancer who will develop a malignancy. Despite these advances in tumor biology, however, controversy continues regarding the surgical procedures that should be performed for the various types of thyroid cancer. Although parathyroid cancer is a very rare but lethal form of neoplasia, benign parathyroid disease is much more common. Recent molecular studies have revealed fascinating differences among the various clinical manifestations of primary hyperparathyroidism.