Neuromyelitis optica (NMO), otherwise known as Devic's disease, is an idiopathic, severe, inflammatory disorder that preferentially affects the optic nerves and spinal cord. Clinical, laboratory and immunopathological evidence suggests that NMO is a humorally mediated disease distinct from MS. A spinal cord lesion extending contiguously over three or more vertebral segments is characteristic of NMO and, in combination with a cranial magnetic resonance imaging scan that does not meet radiological criteria for MS, is over 94% sensitive and 96% specific for NMO diagnosis. The serum autoantibody marker, neuromyelitis optica-immunoglobulin G (NMO-IgG), appears specific for NMO and suggests that the disease spectrum includes cases of recurrent longitudinally extensive transverse myelitis and Japanese opticospinal MS. Its target antigen is the water channel aquaporin-4, suggesting that NMO may represent a novel autoimmune channelopathy. Relapsing NMO has a poor prognosis; therapy, typically with immunosuppression, is necessary as early as possible in the disease course to prevent attack-related disability.