Regeneration is the ability to restore lost or damaged tissues, organs or limbs. It is a common feature in invertebrates, such as worms, but the only vertebrates known to have a significant ability for regeneration is the urodeles. Nevertheless, even humans possess some degree of regeneration ability. Children under 6 years of age are capable of regenerating lost fingertips and the human liver retains its ability to regenerate throughout a person's lifetime. The natural regenerative ability can sometimes be slightly enhanced by physical tension applied to the tissue. The principle is used for techniques such as breast reconstruction and foreskin restoration.

Aside from being used to generally describe any number of specific healing processes, regeneration also is a specific method of healing that is noted for its ability to regrow lost limbs, severed nerve connections, and other wounds that mammals cannot heal. This is present in some animals such as the newt and hydra.

To realize this healing, cells neighboring the injury undergo a process called de-differentiation where the cells revert to a more generalized stem cell like state, similar to that of embryonic cells. In this way, the lost cells are regrown in the same way the original ones were. Some animals like planarians instead keep clusters of non-differentiated cells within their bodies, which migrate to the parts of the body that need healing.

In Urodeles (or salamanders), the regeneration process begins immediately after amputation. Limb regeneration in the axolotl has been extensively studied. After amputation, epidermis covers the stump within 24 hours, forming a structure called the apical epidermal crest (AEC). This results in the formation of a blastema - or a layer of dedifferentiated cells. At the same time, pattern formation genes - such as HoxA and HoxD - are activated as they were when the limb was formed in the embryo. Distal structures such as toes form first from the blastema. Intermediate layers are filled in afterwards. Motor neurons grow with the regenerated limb, and innervate the same target muscles they originally controlled. The entire process takes around 3 month in the adult - and the limb becomes fully functional.

This is not possible in mammals because there are specific factors, still being explored, that inhibit the expression of regeneration. In one test involving rats, blocking two of these factors led to greatly increased nerve cell healing in severed spinal chords. In another, injection of a solution of tissue taken from a newt's blastema into a rat's wound duplicated many of the elements of dedifferentiation.

If the processes behind regeneration are fully understood, it is believed this would lead to better treatment for individuals with nerve injuries, broken backs, paralysis, and missing limbs.