Neuromyelitis optica (NMO) or Devic's disease is an inflammatory neurologic disease characterized by severe optic neuritis and transverse myelitis. Other features of NMO include female preponderance, higher onset age, severe functional disability, longitudinally extensive spinal cord lesions (longer than 3 vertebral segments), and oligoclonal IgG bands negativity. Brain lesions are not uncommon in NMO. The relation between NMO and multiple sclerosis (MS) has long been a matter of controversy, but since the discovery of anti-aquaporin 4 (AQP4) antibody (NMO-IgG), an NMO-specific autoantibody, the clinical, MRI, and laboratory features that distinguish NMO from MS have been clarified. Anti-AQP4 antibody binds to the extracellular domain of AQP4, which is highly expressed in endfeet of astrocytes. Recent neuropathological studies, analysis of CSF-GFAP levels during relapse and experimental studies strongly suggest that NMO is an anti-AQP4 antibody-mediated astrocytopathic disease and that T cell-mediated CNS inflammation is necessary to develop NMO. Also, IL-6 is remarkably elevated in the CSF and appears to regulate plasmablasts to produce anti-AQP4 antibody. Therefore, from the therapeutic point of view, depletion of anti-AQP4 antibody, suppression of T cell response to trigger relapse and anti-IL-6 therapy seem to be pivotal. High-dose intravenous methylprednisolone is the first-line therapy for acute exacerbations of NMO. But plasma exchange should be started soon if corticosteroid is not efficacious. If untreated, AQP4 antibody-positive patients are highly likely to experience relapses within a year. Thus, immunosuppressive therapy (corticosteroids, immunosuppressants, rituximab) should be initiated without delay. Preliminary results suggest that eculizumab, an anti-C5 monoclonal antibody, can also prevent relapse in NMO, Meanwhile, interferon-beta, a first-line disease modifying drug of MS, is not effective in NMO. Symptomatic therapy for pain, paresthesia, spasticity, dysuria and constipation which commonly occur in the chronic stage of NMO is also important to improve patients' quality of life.