First developed for hematologic disorders, the concept of cancer stem cells (CSCs) was expanded to solid tumors, including colorectal cancer (CRC). The traditional model of colon carcinogenesis includes several steps that occur via mutational activation of oncogenes and inactivation of tumor suppressor genes. Intestinal epithelial cells exist for a shorter amount of time than that required to accumulate tumor-inducing genetic changes, so researchers have investigated the concept that CRC arises from the long-lived stem cells, rather than from the differentiated epithelial cells. Colon CSCs were originally identified through the expression of the CD133 glycoprotein using an antibody directed to its epitope AC133. It is not clear if CD133 is a marker of colon CSCs-other cell surface markers, such as epithelial-specific antigen, CD44, CD166, Musashi-1, CD29, CD24, leucine-rich repeat-containing G-protein-coupled receptor 5, and aldehyde dehydrogenase 1, have been proposed. In addition to initiating and sustaining tumor growth, CSCs are believed to mediate cancer relapse after chemotherapy. How can we identify and analyze colon CSCs and what agents are being designed to kill this chemotherapy-refractory population?