According to the different distribution of nevus cells in the skin histology, pigmented nevi are divided into three types: ① Intradermal nevus refers to the nevus cells exist only under the dermis. The appearance is often higher than the skin, hemispherical; the most stable, generally not malignant; ② junctional nevus refers to the nevus cells exist in the epidermis and dermis junction, generally flat, slightly above the skin; junctional nevus is the most active, the most prone to malignant changes. It is usually believed that the pigmented nevus that grows on the palms of hands, finger nails, soles of feet, toes, toenails, eyelids and conjunctiva, lips and mouth, waistline, external genitalia and other parts of the body belong to junctional nevus. These parts are also susceptible to various friction stimuli and have a higher possibility of malignant changes, so they should be taken care of with extra care in life. Mixed nevus refers to nevus cells growing at the junction of epidermis and dermis as well as under the dermis. It has the characteristics of both intradermal nevus and junctional nevus. However, it is sometimes impossible to distinguish between the above three nevi with ordinary naked eyes, and only pathologic section examination can be made to identify them.