Scars are areas of fibrous tissue that can replace normal skin after injury. Scarring is caused by the biological process of wound repair in the skin and other organs and tissues of the body. Therefore, scar formation is a natural part of the healing process. In addition to very small injuries, each wound (for example, after an accident, illness, or surgery) can cause a certain degree of scarring. The exception is a completely regenerated animal that regenerates tissue without scarring.

Scar tissue is composed of the same protein (collagen) as the tissue it replaces, but the fiber composition of the protein is different; the random basket like tissue that replaces the collagen fiber found in normal tissue is formed, in fibrosis, the collagen is cross-linked and forms an obvious arrangement in a single direction. The arrangement of collagen scar tissue is usually inferior to that of normal collagen in terms of functional quality. For example, scars on the skin are less resistant to UV light, and sweat glands and hair follicles do not regrow in scar tissue. Myocardial infarction, usually called a heart attack, can form scars in the heart muscle, resulting in the loss of muscle strength, and may even lead to heart failure. However, some tissues, such as bones, can heal without any structural or functional deterioration.