This cutting-edge review of the current status of research on the early development of lymphocytes emphasizes molecular mechanisms controlling lineage commitment and selection. The reviews explore cellular and molecular aspects of the microenvironment in regulating lymphoid development, discuss ligands and receptors that mediate interactions between progenitor and cells comprising the stromal microenvironment, and demonstrate which are the triggering receptors, signaling pathways, and the developmental checkpoints controlling correct development of B- and T-cells. There is also a comparison of the vital roles of transcription factor combinations in establishing cell identity in the erythroid, myeloid, B, and T lineages. By juxtaposing the insights and approaches normally confined to either immunology or hematology, this important book will help spark the growth of new research initiatives at the intersection of these two fields.