In this review, we discuss recent issues about cancer stem cells, especially in solid tumors. Cancer stem cells show the ability to self-renew; and they possess multi-potential differentiation and tumorigenic potential. Cancer stem cells are reported to show resistance to anticancer drugs and radiotherapy, and therefore, they are involved in progression, recurrence, and metastasis in cancer. The existence of cancer stem cells in leukemia has long been known, since early times. In recent years cancer stem cells have been identified in solid tumors of breast, prostate, lung, brain, colon, and pancreas, owing to the development of identification techniques using cell surface markers. Although many features remain to be addressed, it is expected that cancer stem cells could be a main target for cancer therapy in the near future.