Since the initial discovery of leukemia stem cells nearly a decade ago, a great deal of cancer research has focused on the identification of cancer stem cells (CSCs) in many types of solid tumors, including breast cancer. Through analysis of cell surface markers and xenotransplant models, a subpopulation of putative human breast cancer stem cells (BCSCs) that is CD24-negative/CD44-positive (CD24(-)/CD44+) and bears high aldehyde dehydrogenase 1 activity has been isolated in clinical samples of breast cancer tissues. Human BCSCs are considered to be derived from basal cells that reside in the basal membranes of alveolar units in human adult mammary glands. Furthermore, BCSCs have been shown to express higher levels of oxidative stress-responsive genes, which could confer part of their ability to resist anti-cancer therapy, than non-CSCs. The emerging picture of the biological properties of BCSCs would contribute for devising innovative therapies for breast cancer, targeting the intrinsic and extrinsic factors that maintain the BCSCs.