Hyperparathyroidism is mainly caused by hyperplasia or adenomas of the parathyroid glands. Formerly, the therapeutic strategy depended on the morphological diagnosis (preferentially already made intraoperatively by examination of frozen sections). Today, however, the extent of parathyroid surgery is in many cases guided both by substantial improvements in the preoperative clinical diagnosis as well as the intraoperatively determined decrease in the serum parathyroid hormone level. Nevertheless, the pathologist should be familiar with the differential diagnostic criteria of parathyroid hyperplasia, adenoma, and carcinoma.