Due to the accessibility of the intermediate steps in the progression of colorectal cancer and to the existence of heritable susceptibility to the disease, molecular genetic analysis of colorectal carcinogenesis seems likely to answer many of the questions concerning the fundamental nature of the common human epithelial cancers. Several genetic events appear to be required and, although there is no stringent adherence to any particular sequence of events, the accumulation of genetic defects does show some loose order. Each event must confer growth advantage in order to allow further clonal expansion. Such expansion then makes further events at other crucial loci more likely. Hence, the process proceeds until the tumour is capable of the destructive growth, infiltration and metastasis characteristic of malignancy. This review summarises recent important progress in our understanding of both constitutional and somatic molecular genetic events involved in the development of colorectal cancer. Germline changes responsible for syndromes, such as Familial Adenomatous Polyposis, which result in predisposition to large bowel neoplasia are discussed. The possibility that heritable mutations in tumour suppressor genes might confer susceptibility to apparently sporadic colorectal cancer is proposed.