Cancer is a genetic disease. Colorectal cancer is probably the type of cancer for which the most is known about the genes affected by cancer-causing mutations, their normal functions and their carcinogenic effects when mutated. Most cancer-causing mutations are somatic, occurring in the affected tissue during the course of carcinogenesis. However, most cancers also have a hereditary component that is caused by predisposing mutations that affect the germline, are heritable and contribute to the initiation of carcinogenesis. High-penetrance mutations confer predisposition to colorectal cancer mainly in Lynch syndrome (which involves mutations in mismatch-repair genes) and in familial adenomatous polyposis (which involves mutations in the APC tumour suppressor). Together, these conditions account for 5% or less of all cases of colorectal cancer. Low-penetrance mutations account for a high proportion of all the attributable risk of colorectal cancer, in both familial and sporadic cases. These mutations are more difficult to identify, but mainly due to the implementation of association studies, are increasingly being detected and characterized. The identification of both high- and low-penetrance mutations contributes significantly to our understanding of the molecular genetic processes occurring in cancer. This understanding facilitates the development of therapeutic drugs and preventive strategies.