During the past few decades, technical and conceptual breakthroughs have led to a virtual revolution in developmental biology. In part through cross-species compa- sons and multidisciplinary approaches (combining, for example, classical embry- ogy, genetics, molecular biology, and systems biology), major questions have often been redefined and examined from new angles and with innovative tools. Analyses using such model systems as Drosophila, Xenopus, zebrafish, chick, human, and mouse have underscored the remarkable extent to which molecular and genetic pa- ways are conserved across species and throughout embryonic, fetal, and adult dev- opment. What we learn from the embryo, then, is not only of fundamental interest, but may well have future practical applications in the clinic. A number of excellent volumes, including several in this series (e. g. , Hema- poietic Stem Cell Prools, Klug and Jordan, eds. , 2002), have surveyed methods used in the study of hematopoiesis—the processes by which the multiple lineages of the blood form from stem and progenitor cells during ontogeny and throughout the entire life of the animal. These collections of prools have focused largely on the postnatal cells of mouse and human. Our understanding of hematopoietic devel- ment, however, has benefitted enormously from investigations in a variety of org- isms at different stages of ontogeny.