Breast cancer, one of the most common serious malignancies affecting women, occurs in hereditary and sporadic forms. Hereditary breast cancer accounts for 5–10% of all cases and has some distinctive clinical features compared with sporadic breast cancer. The recently identified genes BRCA1 and BRCA2 appear to account for the majority of hereditary breast cancer in US and European populations. Both of these genes have already been localized and isolate; however, the exact functions of their proteins are not clear yet. The detection of LOH in the 17q21 and 13q12-q13 regions, where the BRCA1 and BRCA2 genes are located, indicates that BRCA1 and BRCA2 act as tumor suppressor genes. The list of identified germline mutations in BRCA1 and BRCA2 is still growing, and mutation carriers have a substantial lifetime risk of both breast and ovarian cancer. However, it is still undetermined whether BRCA1 and BRCA2 play similar important roles in sporadic breast cancer. This paper reviews the current advances in BRCA1/BRCA2 research: the structure of their genes and proteins, their mutation frequencies, their possible roles in both hereditary and sporadic breast cancers, and their functions in transcriptional regulation and DNA repair.