Four types of acanthosis nigricans (a.n.) must be distinguished: "benign" a.n., pseudo a.n., a.n. as part of a syndrome and malignant a.n. These different types affect different age groups, follow different courses, are associated with different conditions, are or are not reversible, have or do not have a genetic basis and also show great differences in regard to their significance for the patient's life or well-being.