One of the most important risk factors for breast cancer is family history of the disease, indicating that genetic factors are important determinants of breast cancer risk. A number of breast cancer susceptibility genes have been identified, the most important being BRCA1 and BRCA2. However, it is estimated that all the currently known breast cancer susceptibility genes accounts for less than 25% of the familial aggregation of breast cancer. In this paper, we review the evidence for other breast cancer susceptibility genes arising from twin studies, pedigree analysis and studies of phenotypes associated with breast cancer, and the progress towards finding other breast cancer susceptibility genes through linkage and association studies. Taken together, the available evidence indicates that susceptibility to breast cancer is mediated through variants in many genes, each conferring a moderate risk of the disease. Such a model of susceptibility has implications for both risk prediction and for future gene identification studies.