Neuromyelitis optica (NMO) is an inflammatory disorder of the central nervous system (CNS) that predominantly affects the optic nerves and spinal cord. Previously, it has been considered to be a severe variant of multiple sclerosis (MS), especially common in Asia. However, the finding that most NMO patients have autoantibodies against aquaporin-4 (AQP4) has improved our knowledge of its pathogenesis and led to the concept that NMO is a disease distinct from MS. Although the 2006 NMO revised criteria are useful for diagnosing NMO, their usefulness in the diagnosis of early-stage NMO is limited. Hence, there is an urgent need for new and more precise diagnostic methods. Interleukin-6 may play important roles in NMO pathogenesis, as it is involved in the survival of plasmablasts that produce anti-AQP4 antibody in the peripheral circulation and in the enhancement of inflammation in the CNS. Severe blood-brain barrier disruption in NMO allows the anti-AQP4 antibody to access the astrocytic endfeet. The anti-AQP4 antibody causes astrocytic damage through complement activation. Thus, NMO is an astrocytopathic, rather than a demyelinating, disease. Some brain lesions specific to NMO have recently been reported. Significant advances in the understanding of NMO pathogenesis are beginning to improve existing treatment strategies and will help develop new treatments. This review focuses on the current advances in NMO research and its clinical characteristics, immunological findings, neuroimaging and pathophysiology.