Primary hyperparathyroidism (PHPT) is a common endocrinopathy, mostly caused by a monoclonal parathyroid adenoma. This review primarily summarizes current knowledge concerning molecular pathogenesis of familial forms of primary hyperparathyroidism and sporadic (non familial) parathyroid tumors. The hereditary syndromes have been recognized as exhibiting Mendelian inheritance patterns and include multiple endocrine neoplasia types 1 (MEN 1) and 2A (MEN 2A), hereditary hyperparathyroidism- jaw tumor (HPT-JT) syndrome, familial isolated hyperparathyroidism (FIHP), familial hypocalciuric hypercalcemia (FHH) and severe neonatal hyperparathyroidism (NSHPT). Inactivating mutations of MEN1 tumor suppressor gene are responsible for MEN 1 in >90% of cases. MEN1 gene has also an established role in the pathogenesis of sporadic parathyroid adenomas. Allelic loss (LOH) of chromosome 11q13 occurs in about 30-40% and somatic mutation of MEN1 gene occur in about 12-20% of sporadic parathyroid adenomas. A mouse model of MEN1 deficiency causes a phenotype that includes the same range of major endocrine tumors as in MEN 1 patients, and exhibits multistage tumor progression with metastatic potential. Hormonal disturbances, such as abnormal PTH and insulin levels, were also observed in these mice. Mutations in a newly identified tumor suppressor gene, HRPT2, have been recently associated with the development of HPT-JT. HRPT2 mutations are also frequent in sporadic parathyroid carcinomas and central to their pathogenesis. MEN1 and HRPT2 genes mutations have also been found in a subset of FIHP families. FHH and NSHPT represent the mildest and severest variants of PHPT, respectively. Both cause hypercalcemia from birth and atypical PHPT that always uniquely persists after subtotal parathyroidectomy. Future identification of additional oncogenes and tumor suppressor genes will clarify the molecular basis of abnormalities of parathyroid proliferation and regulatory function and other specific features unique to the parathyroid tumorigenesis.