Pigmented nevi are caused by abnormal migration of precursor cells from the neural crest to the epidermis during embryonic development, resulting in a localized concentration of melanocytes at the epidermal-dermal junction or in the dermis instead of reaching the basal layer of the epidermis. Most pigmented nevi occur in childhood or adolescence and may appear as macules, papules, papillomas, warts or other forms. They can occur in any area. They can range in size from a few millimeters to several centimeters or larger. The color is often yellowish-brown or black, but can also be blue, purple, or colorless. Pigmented nevi are benign tumors of the melanocyte system and generally do not require treatment. However, pigmented nevi that grow in areas prone to friction can be treated prophylactically. The main treatment methods for pigmented nevi are surgical excision, laser treatment, chemical erosion, liquid nitrogen freezing, etc. Surgical excision is the preferred treatment for pigmented nevus. If other methods cannot completely remove nevus cells at one time, the chance of residual recurrence will increase, and the traumatic stimulation of tissue by laser and freezing treatments may induce malignant lesions. If the following conditions occur, it is necessary to remove the nevus as soon as possible and have the excised pigmented nevus examined pathologically to exclude malignant changes: 1. new pigmented nevus occurs after the age of 30; 2. when the color of a pigmented nevus in one part of the body has a tendency to become darker compared with that of other parts of the body; 3. when it is repeatedly infected or broken; 4. it increases significantly in size within a short period of time; 5. the shape becomes asymmetrical; 6. the surface pigment becomes uneven; 7. the border becomes Irregular; 8. Satellite nevi appear around.