Neuromyelitis optica (NMO; Devic's syndrome or Devic's disease) is a clinically defined disorder consisting of optic neuritis (ON) in combination with myelitis. Its nosological relationship to other central nervous system demyelinating diseases remains uncertain. Advances in understanding of its natural history and pathophysiology may eventually allow determination of whether idiopathic NMO represents a specific disease. This article summarizes recent progress in these areas of NMO research.