Cancer stem cells (CSCs, or tumor initiating cells) are responsible for tumor initiation. If cancer treatment kills most of cancer cells in the stage of transit amplifying and differentiation without killing the stem cells, the surviving CSCs will eventually lead to recurrence of tumors. Studies have suggested that CSCs may be the primary mediators of resistance to chemo- and radio-therapy, leading to failure in cancer therapy. Numerous targets are being investigated for their potential involvement in the self-renewal and chemo- and radio-resistance of cancer cells. However, despite the intensive efforts invested into characterizing the role of cancer stem cells, there is a sense of uncertainty regarding the identity and number of these cells as well as the implications in cancer treatment. In this review, we will discuss the identification of CSCs by cell surface markers, the biology of CSCs, and the role of CSCs in resistance to radio- and chemo-therapy. This review will discuss the advances in targeting CSCs to improve the efficacy of chemo- and radio-therapy.