Thyroid cancer encompasses some forms that practically have no identifiable mortality to one of the most lethal soft tissue cancers known. Because of this variation, a knowledge of the role that age, histology, and extent of disease play in establishing a prognosis is necessary to provide a rational therapeutic program. Younger patients (under age 40) with differentiated cancer, even those with lymph node metastases, should not be overtreated with extensive surgery and 131I or external beam therapy because the prognosis in these patients is so extremely good. Thyroid hormone suppression is probably adequate therapy for patients in this group after obvious disease has been resected. Recurrences can usually be effectively treated with 131I. On the other hand, older patients should be treated more aggressively, especially with the routine use of 131I ablation and therapy after resection of disease. Radiation therapy as described for recurrent disease should be considered at an early point and should be used immediately once the diagnosis of anaplastic cancer has been established. Finally, patience and observation alone should be considered a good therapeutic alternative, for example, in hypercalcitoninemic patients with medullary cancer that has been apparently adequately resected. Repeat operations in these patients fail to eliminate elevated serum calcitonin levels in the majority of cases, and the patients may live for many years with a good quality of life.