Much has been learned in the last two decades about the nature, origin and development of the pigmented nevus.*This has been attained through investigations in comparative embryology, tinctorial histologic studies, and chemical investigations on the nature and elaboration of melanin. However, in spite of this new knowledge there is much uncertainty regarding the clinical pictures associated with the different developmental stages of nevi. This paper is an attempt to relate the clinical types with the different histologic pictures. The need for more clinical knowledge in this field is self-evident. It would enable the clinician to select for complete surgical removal those lesions which in the opinion of most histologists are presumed to have malignant potentialities. In addition, it would provide the physician with criteria for the use of more convenient and sometimes less cosmetically objectionable methods of treatment, such as, for instance, partial