Hematopoiesis is a dynamic process in which eight lineages of mature blood cells are derived from a common stem cell. Great progress has been made in identifying the functionally disparate progenitors that emerge from the stem cell and in elucidating the molecules required for their growth and survival. Further work will be required to understand the molecular mechanisms that regulate commitment of stem and progenitor cells to each stage of progenitor cell development and ultimately into the mature blood cells. Anat Rec Part A 276A:66–74, 2004. © 2004 Wiley-Liss, Inc.