Heredity plays an important causative role in a large percentage of colorectal cancers. Clinical recognition of the hereditary polyposis syndromes, hereditary nonpolyposis colorectal cancer, and common familial colorectal cancer is essential because screening, surveillance, and treatment among affected individuals and their family members differs from that recommended for the general population. More intensive cancer screening and surveillance is required if premature death is to be avoided. Genetic testing is commercially available for most of the hereditary colorectal cancer syndromes and can greatly facilitate the management of patients if properly undertaken.