While germline mutations in BRCA1 and BRCA2 account for most, if not all families with autosomal dominant transmission of susceptibility to both breast and ovarian cancer, it has become clear that together these genes only account for a small proportion of hereditary site-specific breast cancer susceptibility. However, difficulties due to genetic heterogeneity, reduced penetrance and perhaps gene mutation frequency complicate ongoing efforts to identify additional susceptibility genes. Therefore, multiple approaches are being used to identify additional high and low penetrance genes. Families with three or more breast cancer cases are being used in traditional linkage studies, which are expected to yield only moderate or high penetrance susceptibility genes. Breast cancer case-control studies are being used to look for genetic variants or polymorphisms that confer an increased risk of breast cancer in a wide variety of cellular pathways, ranging from the detoxification of environmental carcinogens to steroid hormone metabolism, DNA damage repair and immune surveillance, an approach useful primarily to identify low penetrance susceptibililty genes. However, neither approach has yielded convincing results to date. A third approach, using BRCA1 and BRCA2 mutation carriers to identify genes that are associated with modification of breast cancer risk has met with some limited success, perhaps because effects on breast cancer risk in BRCA1 and BRCA2 mutation carriers are more readily detected in smaller studies, given the much higher number of events in these cohorts at very high risk of breast cancer. Clearly, hereditary breast cancer susceptibility is a complex phenomenon, in which multiple genes may play a role. It will be necessary to use all of these approaches, as well as more comprehensive genomic studies, to identify additional breast cancer-related genes.