Because of their ability to self-renew and differentiate, adult stem cells are the in vivo source for replacing cells lost on a daily basis in high turnover tissues during the life of an organism. Adult stem cells however, do suffer the effects of aging resulting in decreased ability to self-renew and properly differentiate. Aging is a complex process and identification of the mechanisms underlying the aging of (stem) cell population(s) requires that relatively homogenous and well characterized populations can be isolated. Evaluation of the effect of aging on one such adult stem cell population, namely the hematopoietic stem cell (HSC), which can be purified to near homogeneity, has demonstrate that they do suffer cell intrinsic age associated changes. The cells that support HSC, namely marrow stromal cells, or mesenchymal stem cells (MSC), may similarly be affected by aging, although the inability to purify these cells to homogeneity precludes definitive assessment. As HSC and MSC are being used in cell-based therapies clinically, improved insight in the effect of aging on these two stem cell populations will probably impact the selection of sources for these stem cells.