The daily production of hundreds of billions of blood cells through the process of hematopoiesis is a remarkable feat of human physiology. Transport of oxygen to tissues, blood clotting, antibody- and cellular-mediated immunity, bone remodeling, and a host of other functions in the body are dependent on a properly functioning hematopoietic system. As a consequence, many pathological conditions are attributable to blood cell abnormalities, and a fair number of these are now clinically treatable as a direct result of hematopoietic research. Proliferation of hematopoietic stem cells, and their differentiation into the many different lineages of functional mature cells, is highly regulated and responsive to many environmental and physiological challenges. Our relatively advanced understanding of this stem cell system provides potentially important insights into the regulation of development in other tissues, many of which are now being acknowledged as stem ce- based, perhaps even into adulthood. The recent public and scientific fanfare following announcement of human embryonic stem cell studies suggests that stem cell research will continue to be a relevant and exciting topic.