Recently,study on cencer stem cells has been a focus of study.Cancer stem cell is a small population of cencer cells possessing the properties of stem cells：self-renewal,differentiation and proliferation.To date,the existence of cancer stem cells has been proven in acute and chronic myeloid leukemia,breast cancer,brain tumors,liver cancer and colon cancer，etc..In this article we reviews the current progress on cancer stem cells，including the defination,existing evidence,research methods,and challenges in clinical application.