Cancer is considered to be a multifactorial disease in which a host cell is transformed from normal to malignant as a result of complex interactions of external (environmental) stimuli and cancer-predisposing or cancer-suppressing genes. Although certain chemical carcinogens and ionizing radiation are known to cause specific alterations at the level of the gene, other correlations are less clear. Not infrequently, cancer is found to aggregate in families in an apparently nonrandom fashion. It has been through the study of such families that our understanding of the genetic events leading to cancer has developed. Both common and rare tumors may occur together in familial-cancer families. Frequently, tumors occur at an earlier age than one would expect in the general population; often, multiple tumors of different organs develop in a particular affected family member. Recent advances in the genetics of familial cancer syndromes have led to the possibility to perform genetic testing on unaffected relatives who might carry a genetic defect that predisposes them to cancer. Complex ethical, social, and legal implications arise from these new technical advances.