Congenital melanotic nevus (CMN) is caused by a disturbance in the normal growth, development and migration of melanoblasts. The nevus cells are distinguished from and pigment cells by their lack of dendrites. They are usually present at birth or appear within one year after birth. In the latter case, the lesions are generally thought to be present at birth and only gradually appear after birth. The lesions form between 5 and 24 weeks of gestation and are mostly disseminated, brown to dark brown, and may be blue in darker skin tones. They are round or oval with well-defined borders. Most of them are elevated on the skin surface. Adults with a diameter of 20 cm and children with a diameter of 9 cm on the head or 6 cm on the trunk are considered giant nevi. Precise calculation is based on body surface area >2%. This method is difficult to use. About 1% of live births have CMN, but most are small, while the incidence of giant nevi >10cm is about 1/20,000 live births, and larger ones are even rarer, about 1/500,000. CMN features: melanocyte nests are usually located at the epidermal-dermal junction or within the epidermis, and in large CMN, the morphology of nevus cells is usually more variable and complex. Congenital melanotic nevi are characterized by cell nests mostly located in the dermal reticular layer. It is rare for nevus cells to be located in the deeper layers of the skin, except in giant CMNs. large CMNs tend to occur on the trunk, followed by the lower extremities, upper extremities, head, and neck in that order. Giant nevi often span multiple anatomical sites, such as “swimsuit-like or glove-like”, and often have multiple small satellite foci. They may become progressively smaller during the natural course of the disease, but most are darker in color or have hair growths that form plaques or nodules. The differential diagnosis includes, pigmented nevus of the epidermis, sebaceous nevus, milk coffee spot, and Mongolian spot. For conditions that cannot be completely excised at an early stage but need to be viable, include, potential dysplasia or malignant growths such as the presence of ulcers, uneven pigmentation, morphologic changes or the appearance of nodules. The possibility of CMN malignancy into melanoma was known by physicians in the 19th century. 0.5% of melanomas appear before puberty, but 33% of them originate from CMN, and most believe that the tumor originates directly from CMN. there is no evidence that excision reduces the incidence of malignancy. The rate of malignancy: about 0-5% in small ones and 5-10% in large ones. In large CMN, the tumor is often located deep in the epidermal-dermal junction or in other tissues, such as the central nervous system and retroperitoneum, which are more difficult to detect, and often 70% develop within 10 years of age. Large CMNs should be operated on as early as possible and excision should be more complete. Melanoma often occurs on the trunk, head and neck. Clinical manifestations include deepening of pigmentation, accelerated growth, change in shape, nodules, pain, ulcers, and itching, but these symptoms are also often seen in benign CMN, and malignancy may appear later; therefore, long-term follow-up should be performed after resection. In addition to melanoma, large CMN may also present with neurotrophic melanosis, which is an accumulation of melanocytes in the soft meninges that can also become malignant and lead to primary CNS melanoma, in addition to other symptoms such as epilepsy, hydrocephalus and other CNS irritations, or may be asymptomatic. Early MRI scan is recommended for high risk patients. This is especially true in patients with nevi located in the posterior midline with or without satellite foci. Rhabdomyosarcoma may also occur in such patients. In conclusion, all pigmented lesions may develop into melanoma, which may or may not be related to size, but the larger the lesion, the more likely it is to invade deeper tissues. Treatment options: include complete and partial excision, grinding, chemical peeling, laser, etc. The basic principle is to reduce the possibility of malignancy, preserve function, and improve appearance. Laser, grinding, and chemical peeling are used to improve the appearance, but cannot completely remove the nevus cells. It has been reported that lasers increase nevus cell malignancy in vitro, and these methods also make deep nevus cells more difficult to monitor. Complete excision is the only way to reduce malignancy, but sometimes the nevus is so large and invasive to the depths that complete excision is sometimes not possible. In these patients, there is no evidence that prophylactic excision reduces the risk of malignancy. As technology improves, the overall incidence of malignancy is decreasing. Long-term clinical follow-up is important at this time, especially in patients who cannot be excised such as high-risk patients, where moles are large, thick and deeply invasive and early diagnosis is often difficult. Sunlight may cause malignant changes, so for these people, sunlight exposure should be avoided as much as possible. In conclusion, CMN can be divided into two categories, large ones and others. Large ones also have a higher risk of malignant transformation, require early surgery and are more difficult to treat. Therefore the choice of treatment should avoid masking the detection of remaining nevus cells. Laser, grinding, and chemical methods should be used with caution. Excision should try to remove as deep a lesion as possible.