Cancers of the endocrine glands are uncommon. Endocrine tumors—both benign and malignant—although infrequent, present challenges in diagnosis and treatment. Cancers of the thyroid usually do not alter thyroid function, but abnormal hormone secretion can cause the earliest symptoms or signs in cancers of these others endocrine glands. Abnormal plasma levels of these hormones can be useful in monitoring the effects of therapy, and specific inhibitors of hormone biosynthesis or activity can palliate symptoms significantly without having any effect on tumor growth. Diagnosing and treating endocrine cancers requires oncologists, endocrinologists, surgeons, and other subspecialists to work together. This chapter provides a practical guide to the diagnosis and management of endocrine tumors.