The primary function of adult (somatic) stem cells is to maintain tissue homeostasis by replenishing senescent or damaged cells. For example, normal repair and exercise-induced cellular hypertrophy in muscle is mediated by stem cells. Adult stem cells, like other stem cells, are thought to have self-renewal capabilities and the potential to differentiate into specialized cell types. Unlike embryonic stem cells, adult stem cells generally have limited potency and are thought to give rise only to cells within the tissue of residence. However, the potential for adult stem cells to transdifferentiate remains an open area of investigation and has important implications for regenerative medicine. Adult stem cells have been studied in a variety of organs and tissues including the bone marrow, brain, liver, and epithelial tissues such as the skin.