Molecular approaches to the understanding of human neoplastic disease have revealed that multiple genetic alterations are an essential component of tumorigenesis. Both germline and somatic genetic alterations can be involved in the malignant transformation of normal cells. Identification of the genes involved in neoplastic transformation has been approached through the molecular analysis of sporadic cancers and the genetic study of families with an inherited predisposition for cancer. The interplay of these two approaches has led to the characterization of genes such as the retinoblastoma (Rb) gene, the p53 gene and the adenomatous polyposis coli (APC) gene that are all involved in the development of both hereditary and non-hereditary forms of cancer. Inherited mutations in such genes predispose affected families to hereditary cancer syndromes, affording an opportunity to identify genetic lesions that also cause the more common sporadic cancers.