Although the idea was originally proposed decades ago, it has recently emerged that cancer can arise from small cell populations that differ from other progenies. These populations have the ability not only to renew themselves, but also to give rise to diverse phenotypes through the process of differentiation. The result is the formation of clinically observed heterogeneous tumors. Because of their similarity to somatic stem cells, these small cell populations have been termed cancer stem cells (CSCs). The involvement of CSCs was demonstrated in leukemia and has since been noted in other solid tumors, such as brain, breast, and gastrointestinal cancers. We support the notion that a curative operation serves as the most beneficial means to make a prognosis of malignancies. However, currently, chemotherapy is another of the promising strategies for the successful treatment of some malignancies. It is thought, however, that CSCs play a role in resistance to anticancer therapy, thus leading to the occurrence of metastasis, a common characteristic of intractable tumors. As a result, the study of CSCs is expected to improve the effectiveness of current therapies and lead to the development of novel therapeutic approaches for such cancers in the future.