Tremendous progress has been made in recent years in understanding the genetic basis of breast cancer. Epidemiological models have been devised which allow risk estimation for women with one or two relatives with breast cancer. The existence of two autosomal dominant breast cancer susceptibility genes (BRCA1 and BRCA2) have been documented. BRCA1 has been isolated and we are beginning to catalog BRCA1 mutations which appear to be etiologically related to the development of breast cancer in certain families. These BRCA1 mutation studies and the isolation of BRCA2 will allow development of accurate DNA-based testing for the prediction of individual risk in women with a family history of breast cancer. Work by several groups is progressing rapidly on the isolation BRCA2. Ongoing studies investigating the function of BRCA1, and eventually BRCA2, will lead to a better understanding of the role these important genes normally play in preventing the development of breast cancer. Finally, additional breast cancer susceptibility genes are likely to be identified, further expanding the number of tools we have to study, and eventually prevent, breast cancer.