Acute myelogenous leukemias, and perhaps many other cancers, are maintained by a population of cancer stem cells that can regenerate themselves as well as give rise to more differentiated and less proliferative cells that constitute the bulk of the disease. Recent discoveries have shed light on both the nature of leukemia stem cells (LSC) and their cells of origin. Here, we review which hematopoietic cells could give rise to LSC, and the phenotype of fully developed LSC. The perturbed developmental pathways and cellular context of LSC development have implications for the development of new therapeutic approaches.