Nevus is the medical term for a mole.  Many moles are called nevi.  Normal moles are symmetric and have even tone or one single color (usually a light to medium brown) and a shape that is usually oval or round.  Most benign nevi are hereditary.  High numbers of nevi can indicate potential risk for developing melanoma.  It has been shown that increased sun exposure in childhood can influence the number of nevi.  Many moles or nevi can be perfectly normal and never change; however, some may undergo a transformation and become atypical nevi or even mole cancer (melanoma).   Atypical nevi have an irregular shape, color or asymmetry.  Melanoma (mole cancer) often arises within atypical moles, thus it is important to identify atypical nevi and remove them.