Thyroid cancer is the most common endocrine neoplasia. The medullary thyroid carcinoma (MTC) is oneof the most aggressive forms of thyroid malignancy,accounting for up to 10% of all types of this disease. Themode of inheritance of MTC is autosomal dominantly and gain of function mutations in the RET proto-oncogeneare well known to contribute to its development. MTC occurs as hereditary (25%) and sporadic (75%) forms.Hereditary MTC has syndromic (multiple endocrine neoplasia type 2A, B; MEN2A, MEN2B) and non-syndromic(Familial MTC, FMTC) types. Over the last two decades, elucidation of the genetic basis of tumorigenesis hasprovided useful screening tools for affected families. Advances in genetic screening of the RET have enabled earlydetection of hereditary MTCs and prophylactic thyroidectomy for relatives who may not show any symptomsof the disease. In this review we emphasize the main RET mutations in syndromic and non syndromic formsof MTC, and focus on the importance of RET genetic screening for early diagnosis and management of MTCpatients, based on American Thyroid Association guidelines and genotype-phenotype correlation.