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. Also known as somatic stem cells (from Greek Σωματικóς, meaning of the body), they can be found in juvenile as well as adult animals and human bodies.

Scientific interest in adult stem cells is centered on their ability to divide or self-renew indefinitely, and 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 adult stem cells in research and therapy is not considered to be controversial, as they are derived from adult tissue samples rather than destroyed human embryos. They have mainly been studied in humans and model organisms such as mice and rats.

A stem cell possesses two properties:

These properties can be illustrated with relative ease in vitro, using methods such as clonogenic assays, where the progeny of a single cell is characterized. However, it is known that in vitro cell culture conditions can alter the behavior of cells. Proving that a particular subpopulation of cells possesses stem cell properties in vivo is challenging, and so considerable debate exists as to whether some proposed stem cell populations in the adult are indeed stem cells.