Whether neuromyelitis optica (NMO), the co-occurrence of myelitis and optic neuritis, is a variant of multiple sclerosis (MS) or a unique disease is controversial. Distinct neuropathological features and a fulminant clinical course argue in favor of NMO as a distinct disease. However, the combination of neurological impairments of myelitis and optic neuritis occurs in patients with several inflammatory disorders, including multiple sclerosis and collagen vascular diseases. NMO is also associated with certain infectious diseases. The fact that the NMO phenotype occurs in a variety of disease states suggests that NMO does not represent a specific clinical entity. To better understand NMO and its associations with recognized diseases, a systematic review of the literature using MEDLINE was conducted. The history of NMO, its nosology, associations with other diseases, and current concepts of its pathogenesis and treatment is reviewed in this article.