The genetic understanding of colon cancer susceptibility is advancing very rapidly; it has already had a major impact on clinical management, and we have only seen the beginning. Genes responsible for the two well-defined familial colon cancer syndromes (APC and HNPCC) have been identified. These syndromes can now often be diagnosed by genetic rather than by endoscopic screening of the at-risk individuals in a family. Linkage analysis, although clinically cumbersome, can be performed for families with both APC and HNPCC. In some families, the mutation itself can be determined and a specific mutation assay can be used in the other at-risk members of the family. Both linkage analysis and mutational assays are still largely performed in specialized high-risk cancer clinics. For APC, a truncated protein assay performed on cells isolated from peripheral blood is now commercially available, so the genetic diagnosis of APC can be made by any clinician. It is important, however, to be able to provide appropriate genetic counseling to families before and after genetic testing for these familial colon cancer syndromes. The familial clustering of "sporadic" colon cancer has been suggested to be due to the inheritance of a susceptibility gene or genes. A family history of colon cancer has become an important stratification criteria for colon cancer screening programs. Colonoscopic screening is indicated for individuals with two or more FDRs with colon cancer or with one FDR in whom colon cancer developed under the age of 50 or so. If specific susceptibility genes for sporadic colon cancer are identified, it will be possible to target powerful primary preventive and screening programs to this high-risk population (susceptibility gene carriers).