Longitudinal melanonychia (LM), a brown-black band on 1 or multiple nails, is commonly encountered in clinical practice. Benign LM may be due to exogenous (external, blood, bacterial, mycotic) or endogenous (melanin) pigment. Histopathologically, melanin-derived LM may result from overproduction of melanin by a normal number of melanocytes (melanocytic activation) due to physiologic, local, systemic, iatrogenic, syndromic, and drug-induced causes, or from benign (nail matrix nevus and lentigo) or malignant (nail unit melanoma [NUM]) melanocyte hyperplasia. A high index of suspicion is necessary to differentiate benign LM and NUM secondary to similarities in clinical presentation, especially in pediatric patients. Benign pediatric LM may exhibit clinical and onychoscopic features resembling adult NUM; thus, a conservative approach with close follow-up is recommended. Onychoscopy and histopathologic examination of nail clippings are useful initial diagnostic tools for LM, avoiding a biopsy or aiding in biopsy planning and patient triage. Nail matrix excisional biopsy is the gold standard for diagnosing/ruling out NUM. For suspicious LM, a nail matrix tangential excisional biopsy is recommended. A longitudinal excision is recommended for cases with a high-likelihood of invasive NUM, which provides information on tumor extension. Herein, we review the current literature to describe the evaluation and diagnosis of LM.