The investigation of hereditary factors in human cancer was suggested from kindreds that exhibited aggregations of cancer consistent with Mendelian inheritance. A subset of cancer that exhibits strong familial tendencies is due to single genes that 'cause' cancer; more commonly, hereditary factors may influence tumorigenesis in a stepwise probabilistic rather than deterministic manner through a variety of mechanisms, e.g. influencing the disposition of carcinogens. The roles of both common susceptibility genes and rare 'familial' cancer genes are receiving increasing attention in the general population. Population-based studies designed to examine more common genetic variants differ from linkage-based studies. Candidate susceptibility genes may be studied by phenotype or genotype approaches, and the relative advantages and disadvantages of each approach are considered. The issue of gene-environment interaction, implicit in the concept of susceptibility genes, is considered. The influence of genetic factors on individual and attributable risk is addressed.