Neuromyelitis optica (NMO; Devic's disease) and the NMO spectrum disorders are idiopathic inflammatory demyelinating disorders that affect the central nervous system and have a predilection for optic nerves and spinal cord. The identification of NMO-IgG as a disease-specific marker and aquaporin 4 as the target antigen has renewed interest in NMO. Based on current data, we suspect that autoantibodies arising from peripheral B cells bind to aquaporin 4 expressed on astrocyte foot processes on the abluminal surface of microvessels, activate complement and initiate inflammatory demyelination and necrosis. The development of animal models and further analysis of the association of NMO-IgG with disease severity and treatment response will elucidate the pathobiology of NMO.