The hematopoietic stem cells (HSCs) constitute an ideal target for the gene therapy of inherited diseases affecting the hematopoietic system. HSCs, however, constitute a very rare population of progenitor cells, most of which are out of cycle in normal bone marrow. To facilitate their transduction with gammaretro-viral or lentiviral vectors, HSCs are generally enriched using physical or pharmacologic methods. In this chapter we describe efficient procedures which are frequently used to enrich human and mouse HSCs, aiming at the transduction of these cells with adequate gene therapy vectors or the subsequent purification of particular HSCs by fluorescence-activated cell sorting.