Thyroid cancer is not a common disease. It includes tumour types of great diversity in clinical course and molecular basis. Mutations of TSH-receptor, rearrangements of ret proto-oncogene, and altered expression of other tyrosine kinase growth factor receptors are characteristics of the follicular neoplasias and papillary carcinomas, while undifferentiated tumours harbour p53 mutations. Knowledge acquired to date has led to an increased understanding of thyroid growth and tumour development, but it has had no significant impact on diagnostic and treatment measures. On the other hand, the C-cell derived medullary carcinomas include familial cases where identification of germ-line ret mutations provides the basis for prophylactic thyroidectomy in affected individuals.