The cancer stem cell (CSC) concept derives from the fact that cancers are dysregulated tissue clones whose continued propagation is vested in a biologically distinct subset of cells that are typically rare. Rare CSCs have been isolated from a number of human tumors, including hematopoietic, brain, colon, and breast cancer. With the growing evidence that CSCs exist in a wide array of tumors, it is becoming increasingly important to understand the molecular mechanisms that regulate self-renewal and differentiation because corruption of genes involved in these pathways likely participates in tumor growth. Understanding the biology of CSCs will contribute to the identification of molecular targets important for future therapies.