Multiple endocrine neoplasia type 1 (MEN-1) is an autosomal-dominant hereditary disease characterized by the occurrence of tumors of the parathyroids, duodenum and/or pancreas, and anterior pituitary. The syndrome is caused by germline mutations of the MEN1 tumor suppressor gene. The identification of the causative mutations is of paramount importance for the long-term management of affected individuals and their relatives. Multiple endocrine neoplasia type 2 (MEN2) is less frequent than MEN1 and represents a cancer syndrome caused by autosomal-dominant inherited mutations of the RET proto-oncogene, and displays a genotype-phenotype correlation of remarkable clinical relevance. Major components of MEN-2 comprise medullary thyroid carcinoma (MTC), pheochromocytoma, and primary hyperparathyroidism. Since 25-30% of patients with MTC display a hereditary background, genetic testing is indicated once MTC is diagnosed. Occurrence of MTC can be avoided by prophylactic thyroidectomy in early childhood in gene carriers. Early diagnosis and therapy of simultaneous pheochromocytoma avoids the development of complications caused by acute or chronic hypertension.