Bone marrow contains many cell types, including stroma, vascular cells, adipocytes, osteoblasts and osteoclasts, as well as mesenchymal stem cells and hematopoietic stem cells. It was previously thought that cells within bone marrow solely functioned to regenerate cells within the marrow, as well as all circulating hematopoietic cells in peripheral blood. Recent reports, however, suggest that marrow-derived cells can also regenerate other cell types, including cardiac muscle, liver cell types, neuronal and non-neuronal cell types of the brain, as well as endothelial cells and osteoblasts. These multiple cell types could have originated from either of the stem cell populations within bone marrow or potentially other precursors. Therefore, it is not entirely clear whether each of these distinct cell lineages has a true progenitor within marrow or whether the marrow contains a multipotent population of cells that has been set aside during embryogenesis for postnatal repair and remodeling of a variety of tissues. It is clear, however, that directing the fate of bone marrow-derived progenitors (ie toward hematopoietic, vascular or cardiac cell fates) can only be accomplished if the phenotype of the stem cells is defined, and their homing and differentiation programs are elucidated. Much work is focused on these issues, wherein lie the key to harnessing the potential of adult stem cells for autologous cell and gene therapy.