The cancer stem cells hypothesis arises from observation of normal tissue hierarchy and the demonstration of stem cells in normal tissues. Scientists continue to debate whether the putative cancer cells derive from the transformation of normal tissue stem cells or from more differentiated cells. The existence of a subpopulation of tumour cells with stem-cell-like characteristics, including very slow replication and resistance to standard chemotherapy, posses a novel therapeutic challenge. This review summarises the state of development of normal and cancer breast cells and the clinical and therapeutic relevance.