Scars form at the site of an injury to the skin tissue. This is a result of an inflammation response of the body trying to fix the injury. This is a very complex reaction involving many different chemicals and elements, and because of this, the job is not always done right. Sometimes the repair is not done enough with a lack of tissue and appearance of depression; sometimes the healing is over-done with the formation of a raised scar. So scars do come in all shapes and sizes, depending on the wound nature, age, location, nutrition, genetics or infection.

Not all scars are the result of injury. Any skin diseases with an inflammation response may cause the same problem of scarring. For example, acne can cause a variety of scars, chicken pox usually leaves an atrophic scar, and stretch marks are a form of scars caused by excessive stretching. A scar usually takes 12 to 18 months to mature.

There is considerable variation in scarring between one person and another, indicating that some people are more prone to scarring. In the case of keloids, there is a familial tendency.