A dark colored pigmentation of nails can be due to a subungual malignant melanoma. There are, however, many other benign causes and this differentiation is an important challenge in dermatological practice.When should nail pigmentation be clarified by histological investigations?This article gives a survey of the literature on this topic and a review of own experiences with clinical case examples. The various causes of melanonychia are presented and criteria for the early recognition of melanoma of the nail matrix are formulated.The width and depth of color of longitudinal melanonychia do not allow any conclusions on the dignity. If the nail striation is proximally broader than distally, this is evidence of a proliferative event. Confirmation of a subungual hemorrhage does not exclude a malignancy per se. Also detection of pigmentation due to fungi does not exclude the additional presence of a subungual melanoma. A systematic clinical analysis using clear criteria leads to a working diagnosis and suitable treatment.An early and correct designation of nail pigmentation can avoid unnecessary nail interventions but can also be life saving.