Cancer stem cells (CSCs) are a sub-population of cancer cells that are the main tumorigenic cells in tumors. These cells possess characteristics normally associated with stem cells. CSCs have stem cell properties such as self-renewal and the ability to differentiate into multiple cell types, therefore these cells are supposed to be derived from normal adult stem cells or progenitor cells. The cancer stem cell theory suggests that only CSCs within the tumor can self-renew and proliferate extensively to form new tumors. In several different types of cancer, it has been demonstrated that a distinct subset of cells initiates tumors in vivo, whereas the remaining cells do not. Development of flow cytometry, in vitro bioassay, and immunodeficiency mice have contributed to rapid progress in CSC research. CSCs were identified for the first time in human acute myeloid leukemia, in 1997. Thereafter, CSCs have been identified in breast cancer, brain cancer, colon cancer, and pancreatic cancer. As for the head and neck region, in 2007, Prince et al. reported that all cells that have tumorigenic potential were contained within the CD44-Lin population in head and neck squamous cell carcinoma. To date, CSCs have not been found in thyroid cancer, or salivary cancer. Tumors often respond to chemotherapy initially, but frequently recur, suggesting that residual stem cells remaining after therapy are responsible for tumor recurrence. CSCtargeting chemotherapy should lead current cancer treatment to a new stage.