Melanonychia striata is characterized by a tan, brown, or black longitudinal streak within the nail plate that runs from the proximal nail fold to the distal part of the nail plate. Melanonychia striata is due to increased activity of melanocytes or melanocytic hyperplasia in the nail matrix with subsequently increased melanin deposition in the nail plate. The most common cause of melanonychia striata associated with melanocytic activation is ethnic melanonychia which occurs in dark-skinned individuals. Other causes of melanonychia striata related to melanocytic activation include pregnancy, chronic local trauma, infections, medications, dermatological disorders, endocrine disorders, alkaptonuria, hemochromatosis, porphyria, graft-vs-host disease, Peutz-Jeghers syndrome, and Laugier-Hunziker syndrome. Causes of melanonychia striata associated with melanocytic hyperplasia include nail matrix melanocytic nevus, nail lentigo, and nail apparatus/subungual in situ and invasive melanoma. In most cases, melanonychia striata is a benign condition, especially in children. Consequently, most investigators advocate a wait-and-see approach. Nail apparatus/subungual melanoma should be suspected if there is an abrupt onset after middle age, personal or family history of melanoma, rapid growth, darkening of a melanonychia band, pigment variegation, blurry lateral borders, irregular elevation of the surface, a bandwidth >3 mm, proximal widening, associated nail plate dystrophy, single rather than multiple digit involvement, and periungual spread of pigmentation onto the adjacent cuticle and/or proximal and/or lateral nail folds (Hutchinson sign). Prolonged follow-up is mandatory for early detection of possible malignant changes.