Mammary stem cells have recently been identified and purified on the basis of surface antigens and transplantation assays. In addition, recent reports have identified a small sub-population of highly tumorigenic cells within primary and metastatic breast tumors and in a number of breast cancer cell lines. This suggests that, similarly to its normal physiological counterpart, a cancer stem cell may be at the origin of breast cancer. These observations have dramatic biological and clinical implications, as they dictate a revision of our understanding of breast cancer and of our therapeutic strategies. The aim of this article is to review recent data regarding normal mammary epithelial stem cells and evidence in support of the cancer stem cell hypothesis in the breast, and to provide further insight into how taking this subpopulation of cells into account may affect the way we treat epithelial cancers in the future.