Moles are common skin growths that result from a natural process in the skin. Melanin is the natural pigment that gives skin, hair and the irises of the eyes their color. In the skin, melanin is produced in cells called melanocytes located in the two upper layers of the skin. Melanocytes tend to be spread evenly throughout the skin, giving the skin its natural color. When exposed to the sun, melanocytes produce more melanin, darkening the skin with a suntan [source: National Cancer Institute]. When melanocytes don't distribute evenly and instead grow in clusters, moles form [source: Mayo Clinic].
Moles can form anywhere on the body, but they tend to grow more often on areas exposed frequently to the sun, such as the hands, arms, chest, neck and face [source: Cleveland Clinic]. Normally, moles form in the first 20 years of a person's life, though some may grow later. Some moles, especially dysplastic ones, are thought to be influenced by genetics.
Most nevi are a common occurrence that shouldn't cause a person concern. As moles evolve over a person's lifetime, however, they may present problems. Read on to find out how.