Colorectal tumours have proven to be an excellent system in which to identify and study the genetic alterations involved in the development of a common human neoplasm. A prevalent view is that colorectal tumours appear to arise as the result of multiple genetic alterations in the alleles of both oncogenes and tumour suppressor genes. The accumulation of genetic alterations appears to accompany the clinical and biological progression of the tumours and may determine the phenotype of the tumour cells. In addition to the many somatic alterations identified at various stages of colorectal tumour development, recent studies have led to the identification of the adenomatous polyposis coli (APC) gene, which, when mutated in the germline, predisposes to the development of colorectal tumours. On the basis of studies of inherited and somatic mutations in colorectal tumours, a genetic model for colorectal cancer development has been proposed. Although the model is undoubtedly incomplete, it nevertheless provides a useful framework for further studies of the multiple events that underlie human tumour initiation and progression. Numerous questions remain to be answered, including identification of the normal function of the genes implicated in tumorigenesis, how mutations in these genes arise and are selected for and what the relative contribution of the altered genes is to various stages of the neoplastic process. Nevertheless, an optimistic outlook is that fundamental insights into the pathogenesis of human cancer are within our reach.