Hereditary breast and ovarian cancer is among the most commonly encountered adult genetic disease, and it is increasingly important that geneticists, oncologists, surgeons and gynaecologists are aware of the issues regarding risk assessment, prevention and management of women with inherited susceptibility to cancer. Genetic risk can be modified by external factors, but what are these factors, and how might our knowledge of them help us to better define the risks for individual women and to develop strategies for cancer prevention?