This study was undertaken with the hope that some of the rather confused and apparently controversial ideas about moles (nevi) and their relation to cancer and melanomas might be made more readily and more generally understandable. Although the literature is replete with reports on the "common mole," there is little definitely known of its origin, development and relation to melanocarcinoma, because both clinical and histologic opinions on the comparative innocence or malignancy of a particular lesion are often widely divergent. This is discouraging from a practical point of view, as the observer is faced with the unpleasant prospect of a grave prognosis and many uncertainties as to the future course of treatment of what is probably one of the most common growths in the skin. A more positive knowledge of the development and ultimate fate of these lesions is, therefore, essential in order that their removal, which is often solicited