Mismatch repair defects are carcinogenic. This conclusion comes some 80 years after the original description of a type of familial colorectal cancer in which mismatch repair defects are involved, and from decades of dedicated basic science research into fundamental mechanisms cells use to repair their DNA. Mismatch repair (MMR) was described first in bacteria, later in yeast and finally in higher eukaryotes. In bacteria, one of its roles is the rapid repair of replicative errors thereby providing the genome with a 100-1000-fold level of protection against mutation. It also guards the genome by preventing recombination between non-homologous regions of DNA. The information gained from bacteria suddenly became relevant to human neoplasia in 1993 when the RER phenotype of microsatellite instability was discovered in human cancers and was rapidly shown to be due to defects in mismatch repair. Evidence supporting the role of MMR defects in carcinogenesis comes from a variety of independent sources including: (i) theoretical considerations of the requirement for a mutator phenotype as a step in multistage carcinogenesis; (ii) discovering that MMR defects cause a 'mutator phenotype' destabilizing endogenous expressed genes including those integral to carcinogenesis; (iii) finding MMR defects in the germline of HNPCC kindred members; (iv) finding that such defects behave as classic tumor suppressor genes in both familial and sporadic colorectal cancers; (v) discovering that MMR 'knockout' mice have an increased incidence of tumors; and (vi) discovering that genetic complementation of MMR defective cells stabilizes the MMR deficiency-associated microsatellite instability. Models of carcinogenesis now must integrate the concepts of a MMR defect induced mutator phenotype (Loeb) with the concepts of multistep colon carcinogenesis (Fearon and Vogelstein) and clonal heterogeneity/selection (Nowell).