The frequency of malignant melanoma is increasing worldwide and comprises in middle Europe about 10 per 100,000 inhabitants per year; it is fatal in each forth or fifth patient. Risk factors may be endogenous, like fair skin with high sensitivity for sun exposure and the presence of many, possibly atypical ('dysplastic') naevi, or exogenous, e.g. excessive exposure to ultraviolet light. The superficial spreading malignant melanoma is the most common form of clinical manifestation. Applying the A (asymmetry), B (border), C (color), D (diameter), E (elevation) rule is a reliable help for making the diagnosis of malignant melanoma. The differential diagnosis besides various naevi comprises a wide spectrum of pigmented new formations of the skin. Classification of malignant melanoma follows the TNM system; spread, tumor thickness and/or level of infiltration are appropriate parameters. The most important prognostic parameter is tumor thickness, which results in 10-year survival time of only 30% if it exceeds 3 mm. The therapy of malignant melanoma is performed by excision with margins between 1 and 3 cm, depending on tumor thickness. Frequency and type of investigations for follow-up controls depend on the risk for metastases, i.e. on tumor thickness. Best prevention is achieved by avoiding excessive exposure to ultraviolet light, especially during childhood, and by early diagnosis of thin melanoma.