The genetic basis of inherited predisposition to breast cancer has been assiduously investigated for the past two decades and has been the subject of several recent discoveries. Three reasonably well-defined classes of breast cancer susceptibility alleles with different levels of risk and prevalence in the population have become apparent: rare high-penetrance alleles, rare moderate-penetrance alleles and common low-penetrance alleles. The contribution of each component to breast cancer predisposition is still to be fully explored, as are the phenotypic characteristics of the cancers associated with them, the ways in which they interact, much of their biology and their clinical utility. These recent advances herald a new chapter in the exploration of susceptibility to breast cancer and are likely to provide insights relevant to other common, heterogeneous diseases.