Parathyroid carcinoma is a rare tumor that is responsible for <1% of cases of hyperparathyroidism in most parts of the world. An increased incidence of this tumor has been reported in patients with the hyperparathyroidism-jaw tumor (HPT-JT) syndrome, but the etiology of most other cases is unknown. Parathyroid carcinomas tend to occur a decade earlier than adenomas, and the sex ratio approaches unity in contrast to the female preponderance of adenomas. Most patients with carcinomas present with marked hypercalcemia and are more likely to have associated bone and renal disease than those with adenomas. Although fibrosis and mitotic activity are common in carcinomas, these features are not specific for malignancy. The diagnosis of carcinoma should be restricted to those tumors that show invasion of blood vessels, perineural spaces, soft tissues, thyroid gland, or other adjacent structures or to tumors with documented metastases. Mutations of the HRPT2 gene (1q21-q32), which are responsible for the HPT-JT syndrome, have been implicated in the development of a high proportion of parathyroid carcinomas. A subset of patients with mutation-positive carcinomas have germline mutations of the HRPT2 gene. This finding suggests that some patients with apparent sporadic parathyroid carcinomas may have the HPT-JT syndrome or a variant of this syndrome. Because of the high frequency of local recurrence following incomplete excision, an en bloc resection is the preferred surgical approach for treatment of parathyroid carcinomas.