This chapter shows that the individual cell and whole-system renewal is highly complex and involves an astronomical array of biological variables, ranging from how single genes are regulated, to apparently stereotyped patterns of group cell behavior. A Cell replacement is not necessarily the same as cell renewal. Replacement needs to take into account the rates of cell loss and the differentiated cell lineage to which they belong to maintain an accurate balance. Mechanisms, therefore, must exist for C Development of the hematopoietic system has been reviewed in detail. These multipotential stem cells regenerate the entire hematopoietic system and maintain adult hematopoiesis. Despite the apparent blurring of lineage commitment at the stem cell level, lineage fidelity is extremely accurate. Hematopoietic cells can also be characterized according to their adherence to various substrates. Advances in hybridoma technology, coupled with the possibilities offered by flow cytometry and fluorescent-activated cell sorting, have created the most pronounced advances in understanding of the physiology of hematopoiesis.