The hematopoietic system is the adult cellular model in which the biology of the stem cells is the best known and may be a model for numerous other tissues. This model is theoretically based on a hierarchy of cells, which begin on a stem cell that differentiates into mature cells through a large number of cellular stages including hematopoietic progenitors. Hematopoietic stem cells have three main properties: (1) Self-renewal capacities. However, in transplantation experiments this property is limited and may be regulated by the stem cell niche. (2) Multipotentiality. Hematopoietic stem cells are capable of differentiation towards all myeloid and lymphoid lineages. However, recent experiments suggest that, like other somatic stem cells, hematopoietic stem cells are capable of differentiating towards non-hematopoietic lineages. The mechanisms of call commitment towards one cell lineage are beginning to be understood and involve competition between transcription factors. (3) Migration. Stem cells are able to travel through the blood. These migration capacities may be an important but underestimated role in the function of hematopoietic stem cells. Mobilization of stem cells by growth factors is now clinically used for hematopoietic transplantation in man. CXCR4 and its ligand, the SDF-1 chemokine, play an important role in stem cell chemotaxis and in their retention in the marrow.