A fundamental problem in cancer research is identification of the cells within a tumor that sustain the growth of the neoplastic clone. The concept that only a subpopulation of rare cancer stem cells (CSCs) is responsible for maintenance of the neoplasm emerged nearly 50 years ago; however, conclusive proof for the existence of a CSC was obtained only relatively recently. The evidence for the existence of CSCs was first derived from the study of human acute myeloid leukemia (AML), largely because of the availability of quantitative stem cell assays for the leukemic stem cell (LSC). These studies showed that only rare cells within the leukemic clone had the capacity to initiate AML growth after transplant into NOD/SCID mice, establishing the hierarchical organization of AML. Recent clonal-tracking studies showed that the LSC compartment is composed of different classes of LSCs, which can be distinguished on the basis of self-renewal potential. These findings have important implications for our understanding of the leukemogenic process as well as the design of more effective therapies to eliminate AML based on eradication of the LSCs. These studies are briefly reviewed here.