Recently it is considered that there is a small population of cells with stem cell property not only in leukemia but also in solid cancer.These cells show the ability of self-renewal and multi-potential differentiation, and can initiate and maintain a tumor. The origin of cancer stem cells might be their normal stem cells, progenitor cells, or bone marrow-derived cells. It is still difficult to isolate cancer stem cells in solid cancer. There are three possible methodologies to isolate or identify cancer stem cells; the use of a surface marker, use of cells cultured in a specific condition (sphere), or the use of side population cells identified by FACS. The gold standard assay that fulfills the definition of cancer stem cell may be serial transplantation in animal models. Cancer stem cells are likely to be responsible for disease relapse or metastasis, and also for resistance to radiation or conventional chemotherapy. The stem cell niche plays an important role on maintaining cancer stem cells. The novel promising therapies against cancer stem cells are considered, including antibody-based therapy, signal inhibitors, overcoming radiation and drug resistance, or differentiation therapy. Another interesting therapy targeting the niche may also be considered.