From the cytological classification, pigmented nevi can be divided into two categories: non-cellular nevi and cellular nevi. 1.Non-cellular nevus: Non-cellular nevus can be divided into two kinds, one is called freckle and the other is called pigmented nevus. It is a kind of yellowish-brown, tiny rash, which occurs on the face and other exposed parts of the body, often related to sunlight exposure. Some young girls’ freckles are very obvious in the spring and summer, but most of them fade away or are not obvious after the fall, which often brings them troubles. Age spots on the face or arms of the elderly also belong to this category. Freckles are only pigmentation in the epidermal basal layer of the skin, and no nevus cells are seen. Pigmented spots are dark patches on the surface of the skin that are larger than freckles, in small patches or large areas. Some of them are detected at birth, while others are deepened and discovered as the child grows up. Pigmented spots are only pigmentation on the surface of the skin, and the problem of malignant change. 2.Cellular nevus: Classified from the tissue structure of skin cells, cellular nevus can be divided into five different types, namely, intradermal nevus, junctional nevus, mixed nevus, nevus of lan and juvenile nevus. Among these five types of nevi, the latter four have the tendency of malignant transformation and should be noted. However, the first three are usually found on the surface of human skin, namely, intradermal nevus, junctional nevus and mixed nevus. They have their own characteristics in terms of size, color, location, morphology and histological structure, etc. The following is a brief description and identification. Intradermal nevus: 1. Size: limited granules or large black spots of 1-2mm. 2. Color: no pigmentation change, or from light brown, gray-black to dark black. 3. 4.Morphology: The surface is flat or slightly higher than the skin surface, often with hair growth. 5. Histological structure: nevus cells are in the dermis, and nevus cells contain melanin. 6. 5. Histological structure: active nevus cells at the epidermal-dermal junction 6. If the color deepens, the lesion increases, or satellite spots and surface rupture appear, early treatment is needed. Mixed nevi: 1. Vitality of junctional nevus, possibility of malignant transformation