MSCs are multipotent cells that can be isolated from several human tissues and expanded ex vivo for clinical use. They comprise a heterogeneous population of cells, which, through production of growth factors, cell-to-cell interactions and secretion of matrix proteins, has a role in the regulation of hematopoiesis. In recent years, several experimental studies have shown that MSCs are endowed with immunomodulatory properties and with the capacity to promote graft survival in animal models. In view of these properties, MSCs have been tested in pilot studies aimed at preventing/treating graft rejection and at accelerating recovery after hematopoietic cell transplantation (HCT). The available clinical evidence deriving from these studies indicates that MSC infusion is safe and promising in terms of capacity of preventing graft failure. More debated is the effect of MSCs for what concerns their capacity of accelerating hematopoietic reconstitution after HCT. Whether the favorable effect of MSCs largely depends on the type of transplantation remains also a field of future investigation. Moreover, future researches should be oriented to gain more insights on MSC biological and functional mechanisms relevant for exploiting their use in the modulation of alloreactivity and in the promotion of hematopoietic reconstitution.