For many years the mammalian blood cell system has provided cell biologists and haematologists with one of the best experimental models in which to unravel how one stem cell -- the hematopoietic stem cell -- gives rise to many different types of progeny. Numerous models of lineage relationships have emerged, but the most influential of these, in which differentiating cells undergo a series of binary choices, has been increasingly challenged in recent years -- to the extent that the accumulation of new findings recently culminated in a Nature commentary suggesting that "the latest research will necessitate revision of textbook accounts". This book brings together contributions from many leading experts in the field of blood cell development who each discuss both the overall process of hematopoiesis and the origins and development of each of the cells of the blood and immune systems. It describes how new molecular, cellular and -- particularly -- transgenic tools are helping us understand the processes that control the lineage fates of hematopoietic stem and progenitor cells and how lineage-committed progeny develop along particular maturation pathways.