The mammary epithelium contains multipotent stem cells that give rise to all differentiated cell types present within the tissue. Mammary epithelial stem cells have been prospectively purified from dissociated mammary epithelium on the basis of cell surface antigen expression. It has become apparent in recent years that for breast cancer and other malignancies only a small proportion of tumour cells--'cancer stem cells'--have the capacity for extensive proliferation and transferral of the tumour. We review the evidence for breast cancer stem cells, we consider their relationship to mammary epithelial stem cells and we examine the implications for current and future therapeutic strategies.