All authors integrating the known facts into a model of thyroid carcinogenesis concur that two main histotypes of thyroid cancer exhibit different routes of molecular development. RET rearrangements are an initiating event in papillary carcinoma, and simultaneously the most characteristic mutation for this type of cancer. They are followed by further, not well recognized, mutations. RAS mutations are regarded as a crucial event in the development of follicular tumors already at the adenoma step, while in papillary cancer they belong to the spectrum of secondary mutations, enabling tumor progression. Aberrant DNA methylation, causing loss of P16 tumor supressor gene, may be a common event in both types of cancer. Aneuploidy is seen much more frequently in follicular than in papillary cancer, which also exhibits a low rate for loss of heterozygosity and microsatellite instability. Mutations of the P53 tumor supressor gene are a common feature of undifferentiated thyroid cancers and could be responsible for their aggressive phenotype. RET rearrangements have been proposed as identifying fingerprints for irradiation induced thyroid cancer in children. Our own data speak against this hypothesis. We noted a high frequency of RET/PTC3 mutations in a group of Polish children with papillary thyroid carcinoma, regarded as sporadic cancer.