Cancer is a disease resulting from complex patterns of genetic alterations that lead to malignant transformation. As such, cancer is a genetically determined disease. The vast majority of cancers are sporadic and non-heritable: genetic alterations are acquired during life, notably by exposure to genotoxic agents. However, most cancer types also have hereditary forms, reflected in extreme situations by family clusters. These forms, which represent 2 to 5% of all cancers, result from transmission of germline mutations which considerably increase the risk of cancer. Over the past fifteen years, hereditary cancers have raised much interest in the scientific and medical communities.