The study of thyroid tumor genetics has great relevance to surgeons and facilitates understanding tumor pathogenesis, prediction of tumor behavior, and management decisions. The genes implicated can be broadly categorized as oncogenes or tumor-suppressor genes. The RET oncogene has well established roles in the development of both papillary (PTC) and medullary (MTC) thyroid carcinoma. Genetic screening for germline RET mutations in members of multiple endocrine neoplasia type II (MEN-II) families is now widely performed, and prophylactic thyroidectomy in gene carriers is advisable at an early age. Patients with apparently sporadic MTC can also be screened to rule out familial disease. The demonstration of a RET rearrangement in a patient's PTC may have prognostic significance, but as yet there are no management implications. The thyrotropin receptor (TSH-R) and Gsalpha become oncogenic through point mutation and are associated with the development of toxic thyroid adenomas. The ras oncogene is implicated in the early stages of development of several thyroid tumor types. Tumor-suppressor genes also have a role in thyroid tumor formation. The p53 gene appears to be involved in the process of transformation to the anaplastic phenotype and the PTEN gene in the development of follicular adenomas but not carcinomas. There is still limited evidence for the so called adenoma-carcinoma sequence of the thyroid follicular cell. Loss of heterozygosity studies have enabled identification of tumor-suppressor genes, and their findings suggests differences in the pathogenesis of PTCs compared with follicular cancers. Surgical decision-making will benefit from these basic molecular advances, which rapidly translates into improved patient management.