Colon cancer is a common disease that can be sporadic, familial, or inherited. Recent advances have contributed to the understanding of the molecular basis of these various patterns of colon cancer. Germline genetic mutations are the basis of inherited colon cancer syndromes; an accumulation of somatic mutations in a cell is the basis of sporadic colon cancer; and, in Ashkenazi Jewish persons, a mutation that was previously thought to be a polymorphism may cause familial colon cancer. Mutations of three different classes of genes have been described in colon cancer etiology: oncogenes, suppressor genes, and mismatch repair genes. Knowledge of many of the specific mutations responsible for colon carcinogenesis allows an understanding of the phenotypic manifestations observed and forms the basis of genetic testing for inherited disease. Although genetic testing is possible and available, it is only an adjunct to the clinical management of persons at risk for colon cancer and patients with colon cancer. As a result of advances in the understanding of the molecular causes of colon cancer and the availability of colon cancer screening methods such as colonoscopy, it should be possible to prevent the vast majority of colon cancer in our society. Practicing clinicians should recognize the patterns of clinical colon cancer, understand its causes, and be able to use genetic testing and endoscopic screening for prevention.