The biggest challenge for cancer research is relapses that occur in patients undergoing chemotherapy and radiotherapy, suggesting that some cells in tumors escape targeted treatment. Key questions are why relapses occur and why current therapies fail to remove all cancer cells. The cancer stem-cell hypothesisis based on the fact that not all cells within a tumor are similar. Other than tumorigenesis and metastasis, cancer stem cells have some properties that are similar to those of normal stem cells, such as self-renewal and differentiation. Accordingly, breast cancer stem cells may arise from mutation of normal mammary stem cells or progenitor cells. Cancer stem cell regulation involves several factors, such as Wnt, Notch, and Hedgehog, mutations of which endow cancer stem cells with the capacity for self-renewal. Moreover, epithelial mesenchymal transition and microRNAs recently have been shown to regulate the "stemness " of cancer cells. Targeting cancer stem cells could prevent relapse and provide new hope for cancer prevention.