Adult stem cells are present in most postnatal tissues of mammals. Tissues with high rates of cell turnover depend on the functional capacity of stem cells for lifelong maintenance of tissue homeostasis. Adult stem cells are also required for the regeneration of tissues in response to injury as in, for example, the regeneration of skeletal muscle. In addition to its function in tissue homeostasis and regeneration, adult stem cells can represent the cell type of origin of various types of cancers including leukemia and colorectal cancer. Stem cells are the most long-lived cells in the proliferative compartment of mammalian tissues. Therefore, stem cells have an increased risk of acquiring mutations that could ultimately lead to the transformation of tissue stem cells. This publication presents the current knowledge in the field of stem cell aging, which was discussed at the Else Kröner-Fresenius Symposium on Advances in Stem Cell Aging in 2011. It will be of special interest to scientists working on stem cell research, aging, regeneration, and cancer as well as physicians and scientists specializing in geriatric medicine, internal medicine, and surgery.