Neuromyelitis optica (NMO) is an inflammatory demyelinating disease of the central nervous system that selectively targets the optic nerve and spinal cord, although it may also target certain areas of the brain. The majority of cases are associated with relapses. A specific biomarker, an autoantibody that targets aquaporin-4, is present in the majority of patients and facilitates the diagnosis. Detection of this biomarker in serum has enabled recognition of an expanded spectrum of clinical disorders that previously would not have met diagnostic criteria for NMO. Aquaporin-4 IgG1 autoantibodies are pathogenic and produce lesions of the brain when injected intracerebrally or systemically. The clinical course of NMO is dominated by acute attacks. Progressive worsening of disability, as occurs in prototypic multiple sclerosis, is distinctly unusual. Corticosteroids and plasma exchange are useful for management of acute attacks. Several treatments used to prevent attacks of multiple sclerosis are ineffective in this condition; effective immunotherapies include azathioprine, mycophenolate mofetil and rituximab.