Moles (nevi) are growths on the skin. Moles are very common. Most people have between 10 and 40 moles. A person may develop new moles from time to time, usually until about age 40.
About one out of every ten people has at least one unusual (or atypical) mole that looks different from an ordinary mole. Dysplastic nevi are more likely than ordinary moles to develop into a type of skin cancer called melanoma. Because of this, moles should be checked regularly by a doctor, especially if they look unusual; grow larger; or change in color, outline, or in any other way.