Genetic factors can dramatically influence the risk of colorectal cancer, and the molecular bases of many hereditary colorectal cancer syndromes, including familial adenomatous polyposis (FAP), attenuated FAP (AFAP), and hereditary nonpolyposis colorectal cancer (HNPCC) have been elucidated. Additional syndromes continue to be defined as new genes, including MYH , are linked to the development of colonic polyps and cancer. The risks of colorectal cancer are variable and depend on the specific germline alterations. Some mutations are associated with a 100% lifetime risk of developing cancer, while others are associated with only a mild increase in risk. Although there are overlapping clinical features in many of these syndromes, they can be distinguished by the age at cancer diagnosis, inheritance pattern, number and distribution of polyps, specific histologic features of the cancers, and the presence of distinctive extracolonic features. The introduction and refinement of genetic testing has provided a new and invaluable tool for the diagnosis and assessment of cancer risk for suspected cases of hereditary colon cancer.