Longitudinal melanonychia (LM) may represent nail matrix melanocyte activation (defined as a normal number of melanocytes with increased production of melanin), benign hyperplasia, or melanoma, in addition to multiple nonmelanocyte pathologies, including hemorrhage and infection. This article details an algorithmic approach to LM, including a careful history and physical examination, dermoscopy, and ability to sample the matrix using 3 biopsy techniques, a 3-mm punch excision, a lateral longitudinal excision, and a matrix shave biopsy. Facility with all 3 techniques will allow the physician to procure appropriate nail matrix specimens for diagnosis.