The theory that cancer may be originated and sustained by a small proportion of stem-like, self-renewing cells (termed 'cancer stem cells') has gained support in recent years. Breast cancer stem cells have been identified as CD44+CD24- breast tumour cells and have recently been isolated and propagated in vitro. It has been demonstrated that these cells exclusively retain the ability to form new tumours in mouse models and that they display stem/progenitor cell properties. The ability to identify breast cancer stem cells in vivo and to propagate them in vitro provides the means to compare them with normal cells, in order to investigate from which cell they originate, which molecular alterations critically affect them, and how they interact with the microenvironment. Elucidation of these critical points is essential to develop new therapeutic strategies and to improve diagnosis and prognosis for breast cancer patients.