The main role of tumour suppressor genes is the inhibition of cell proliferation. Somatic mutations in these genes are found frequently in sporadic tumors. Germ line mutations in tumour suppressor genes are responsible for hereditary cancer syndromes. In a carrier of such a germ line mutation, a somatic mutation or loss of the remaining functional copy of the gene is sufficient for the complete loss of function of the tumour suppressor. Therefore the carriers of germ line mutations have a high risk of developing malignancies. Many tumour suppressor genes have been cloned and characterized recently and many others are intensively searched for. Protein products of these genes serve different cellular functions and many of them directly participate in the cell cycle control. The characterization of tumour suppressor genes is important both for the understanding of processes of carcinogenesis and for practical use in the diagnostics, prognostics and therapy of tumours.