Allogeneic donor blood cells and autologous peripheral blood leukocytes (PBL), obtained following �clinical mobilization procedures, are routinely used as a major source of hematopoietic stem and progenitor cells (HSPC) for transplantation protocols. It is, therefore, essential to evaluate and to quantify the extent by which the HSPC are mobilized and enriched in the circulation in correlation with their long-term hematopoietic reconstitution capacity. In this chapter, we describe quantitative methods that measure the number of mobilized HSPC according to specific criteria, as well as their functional properties in vitro and in vivo . The described assays are useful for assessment of progenitor cell mobilization as applied to both human and murine HSPC.