Every day, the body produces billions of new blood cells. Each of these is derived from a rare cell in the bone marrow called the hematopoietic stem cell (HSC). Because most mature blood cells have a limited lifespan, the ability of HSCs to self-renew and replenish the mature cell compartment is critical to sustaining life. While great progress has been made in isolating HSCs and defining their functional and phenotypic characteristics, the molecular mechanisms that regulate their self-renewal remain a mystery. Over the last few years, alterations in HSC frequency and self-renewal capacity in transgenic and knock-out mice have led to the identification of novel mediators of HSC homeostasis in vivo. These genetically modified mice have revealed that maintenance of survival, proliferation, quiescence, and normal telomere length all contribute to the self-renewal of HSCs. They also highlight the need to test in context of the normal microenvironment the role of signaling molecules such as Notch and Wnt, which have emerged recently as important regulators of HSC self-renewal. The emerging picture these data provide of the regulation of self-renewal in HSCs has provided a better understanding of the basic biology of stem cells and holds promise for designing strategies to improve bone marrow transplantation.