Approximately 10% of thyroid cancers are either poorly differentiated epithelial tumors or nonepithelial malignancies that are included in the World Health Organization (WHO) classification of thyroid tumors.1 The variants of epithelial tumors, including anaplastic, squamous cell, tall cell, insular, and other poorly differentiated types of thyroid cancer, are characterized by more aggressive courses than are well-differentiated histologic subtypes. In addition, nonepithelial malignancies (e.g., lymphomas, sarcomas, and others) and tumors that metastasize to the thyroid occur and can now be recognized by fine-needle aspiration (FNA). The clinician must recognize these uncommon tumors because they are treated differently from the typical epithelial thyroid cancers. The clinical presentation, treatment, and prognosis of these unusual thyroid malignancies are discussed in this chapter. More common thyroid malignancies are discussed in Chapter 40.