Germline mutations in the BRCA1 and BRCA2 genes are the most important known causes of inherited susceptibility to breast and ovarian cancer. In this chapter, we outline the research leading to the identification of these genes. We discuss the risks of breast, ovarian and other cancers conferred by mutations in these genes; their contribution to breast and ovarian cancer incidence; and the pathology and clinical outcome of these tumours. We also discuss the role of genetic and lifestyle modifiers of risk in mutation carriers and, briefly, the function of these genes. Management of BRCA1 and BRCA2 mutation carriers is discussed in more detail in Chapters 21-23.