Longitudinal melanonychia presents a difficult clinical challenge because subungual melanoma must always be included in the differential diagnosis and because the cause of longitudinal melanonychia is usually not apparent. Accordingly, biopsy is often necessary to establish the cause. This review attempts to expedite management by providing suggestions for the examination of patients with this disorder. The causes of longitudinal melanonychia are enumerated and clues to arrive at the various causes are discussed. Similarities between longitudinal melanonychia and subungual melanoma are discussed in an effort to clarify their differences and similarities; clues to the diagnosis of subungual melanoma are also discussed. Various biopsy techniques applicable to longitudinal melanonychia are considered and the indications for different surgical approaches are emphasized. The importance of the pathologist in interpreting nail biopsy specimens is emphasized.