Book excerpt: The hematopoietic system plays roles that are crucial for survival of the host: delivery of oxygen to tissues, arrest of accidental blood leaking from blood vessels, and fending off of invading microbes by humoral, cell-mediated, and phagocytic immunity. The activity of the hematopoietic system is staggering: daily, a normal adult produces approximately 2.5 billion erythrocytes, 2.5 billion platelets, and 1 billion granulocytes per kilogram of body weight. This production is adjusted in a timely fashion to changes in actual needs and can vary from nearly none to many times the normal rate depending on needs which vary from day to day, or even minute to minute. In response to a variety of stimuli, the cellular components of the blood are promptly increased or decreased in production to maintain appropriate numbers to optimally protect the host from hypoxia, infection, and hemorrhage. How does this all happen and happen without over or under responding? There has been extraordinary growth in our understanding ofhematopoiesis over the last two decades. Occupying center stage is the pluripotent stern cell and its progeny. Hematopoietic stern cells have been characterized by their capacity for self renewal and their ability to proliferate and differentiate along multiple lineages. Few in number, the stern cell gives rise to all circulating neutrophils, erythrocytes, lymphoid cells, and platelets. In hematopoietic transplantation, the stern cell is capable of restoring long-term hematopoiesis in a lethally irradiated host.