This chapter discusses tumors of the parathyroid gland, where two distinct tumors of the parathyroid gland are adenoma and carcinoma. Both parathyroid adenoma and carcinoma present in the setting of primary hyperparathyroidism. Chromosomal abnormalities are described with parathyroid tumors, where both parathyroid adenoma and carcinoma are functional tumors in that they produce parathyroid hormone. Renal manifestations of hyperparathyroidism are seen in less than 20 percent of patients with parathyroid adenoma. Parathyroid carcinomas tend to be grey, firm, lobulated and have a dense fibrous capsule. Tumor markers may aid in the histologic diagnosis of parathyroid carcinoma, and are associated with parathyroid carcinoma include proliferating cell nuclear antigen (PCNA) and Ki-67. A major limitation of sestamibi scans is that coexistent thyroid nodules or other metabolically active tissues can mimic parathyroid adenomas and cause false-positive results.