Multiple endocrine neoplasia Type 2 is a rare familial cancer syndrome transmitted in an autosomal dominant manner. It is characterized by the association of medullary thyroid carcinoma with pheochromocytoma and hyperparathyroidism. Medullary thyroid carcinoma, present in virtually all patients, is the principal cause of death. In 1993, germline mutations in the RET proto-oncogene were identified as the underlying cause of the syndrome. Genetic screening of at-risk family members can now be performed with high specificity and sensitivity. The ability to determine gene carrier status at a preclinical stage is of great value as it allows early prophylactic thyroidectomy. The specific RET codon mutation correlates with clinical variants of the syndrome, age at onset and aggressiveness of medullary thyroid carcinoma. This review will focus on mutational spectrum, genotype-phenotype correlations and clinical decisions based on genetic information.