The transformation of the normal fully differentiated thyroid follicular cell to the rapidly growing undifferentiated anaplastic thyroid carcinoma cell involves a number of stages which have been defined morphologically and are now being related to various growth pathways and to molecular biological defects. The two main factors involved in this transformation are growth stimulation and mutagenesis. Growth stimulation alone, through elevated TSH, can lead to the development of thyroid tumours, usually benign, and retaining TSH dependency in some cases. Mutagens alone, if growth is suppressed, do not produce tumours, the combination of mutagens and increased growth is a potent carcinogenic regime. Non-genotoxic carcinogenesis in the thyroid involves growth, without mutagenesis the agent often causes this through affecting one component of thyroid hormone synthesis or metabolism, leading to a fall in thyroid hormone levels and a rise in TSH. Growth stimulation increases the rate of cell division, and therefore increases the chance of a mutation. Continued growth increases the change of subsequent events, in particular loss of heterozygosity in a tumour suppressor gene. The main oncogenes involved in human thyroid carcinogens are ras in the follicular tumour pathway, and ret in the papillary carcinoma pathway. p53 is involved in the progression of either papillary or follicular adenoma to an undifferentiated carcinoma. In experimental thyroid carcinogenesis, ras is again involved, with a link between the mutagenic agent used and the type of ras gene showing mutation. Analysis of the involvement of different growth factors and oncogenes in thyroid carcinogenesis suggests that genes related to the two receptors concerned with normal TSH stimulated growth, TSH receptor and the IGF1 receptor may be involved in the progression of thyroid tumours of follicular pathology. Several tyrosine kinase receptors with unknown ligands or of uncertain physiological function are linked to papillary carcinoma. The recent large increase in papillary carcinoma of the thyroid in children exposed to fallout from the Chernobyl nuclear accident underlines the importance of understanding the pathobiology of thyroid neoplasia.