Adult stem cells are undifferentiated cells, found throughout the body after development, that multiply by cell division to replenish dying cells and regenerate damaged tissues. They are also known as somatic stem cells (from Greek σωματικóς, meaning of the body). Unlike embryonic stem cells, they can be found in juvenile and adult animals, including humans.

Scientific interest in adult stem cells is centered around two main characteristics. The first of which is their ability to divide or self-renew indefinitely, and the second their ability to generate all the cell types of the organ from which they originate, potentially regenerating the entire organ from a few cells. Unlike embryonic stem cells, the use of human adult stem cells in research and therapy is not considered to be controversial, as they are derived from adult tissue samples rather than human embryos designated for scientific research. The main functions of adult stem cells are to replace cells that are at risk of possibly dying as a result of disease or injury and to maintain a state of homeostasis within the cell. There are three main methods to determine if the adult stem cell is capable of becoming a specialized cell. The adult stem cell can be labeled in vivo and tracked, it can be isolated and then transplanted back into the organism, and it can be isolated in vivo and manipulated with growth hormones. They have mainly been studied in humans and model organisms, such as mice, rats and planarians.

A stem cell possesses two properties:

To ensure self-renewal, stem cells undergo two types of cell division (see Stem cell division and differentiation diagram). Symmetric division gives rise to two identical daughter stem cells, whereas asymmetric division produces one stem cell and one progenitor cell with limited self-renewal potential. Progenitors can go through several rounds of cell division before finally differentiating into a mature cell. It is believed that the molecular distinction between symmetric and asymmetric divisions lies in the differential segregation of cell membrane proteins (such as receptors) and their associated proteins between the daughter cells.

Under normal conditions, tissue stem cells divide slowly and infrequently. They exhibit signs of quiescence or reversible growth arrest. The niche the stem cell is found in plays a large role in maintaining quiescence. Perturbed niches cause the stem cell to begin actively dividing again to replace lost or damaged cells until the niche is restored. In hematopoietic stem cells, the MAPK/ERK pathway and PI3K/AKT/mTOR pathway regulate this transition. The ability to regulate the cell cycle in response to external cues helps prevent stem cell exhaustion or the gradual loss of stem cells following an altered balance between dormant and active states. Infrequent cell divisions also help reduce the risk of acquiring DNA mutations that would be passed on to daughter cells.

Discoveries in recent years have suggested that adult stem cells might have the ability to differentiate into cell types from different germ layers. For instance, neural stem cells from the brain, which are derived from ectoderm, can differentiate into ectoderm, mesoderm, and endoderm. Stem cells from the bone marrow, which is derived from mesoderm, can differentiate into liver, lung, GI tract, and skin, which are derived from endoderm and mesoderm. This phenomenon is referred to as stem cell transdifferentiation or plasticity. It can be induced by modifying the growth medium when stem cells are cultured in vitro or by transplanting them to an organ of the body different from the one they were originally isolated from. There is yet no consensus among biologists on the prevalence and physiological and therapeutic relevance of stem cell plasticity. More recent findings suggest that pluripotent stem cells may reside in blood and adult tissues in a dormant state. These cells are referred to as "Blastomere Like Stem Cells" (BLSCs) and "very small embryonic-like" (VSEL) stem cells, and display pluripotency in vitro. As BLSCs and VSEL cells are present in virtually all adult tissues, including the lungs, brain, kidneys, muscles, and pancreas, co-purification of BLSCs and VSEL cells with other populations of adult stem cells may explain the apparent pluripotency of adult stem cell populations. However, recent studies have shown that both human and murine VSEL cells lack stem cell characteristics and are not pluripotent.

Stem cell function becomes impaired with age, and this contributes to progressive deterioration of tissue maintenance and repair. A likely important cause of increasing stem cell dysfunction is an age-dependent accumulation of DNA damage in both stem cells and the cells that comprise the stem cell environment. (See also DNA damage theory of aging.)

Adult stem cells can, however, be artificially reverted to a state where they behave like embryonic stem cells (including the associated DNA repair mechanisms). This was done with mice as early as 2006 with prospects to slow down human aging substantially. Such cells are one of the various classes of induced stem cells.