This chapter discusses the history, epidemiology, clinical presentation, pathogenesis, and treatment of neuromyelitis optica (NMO). NMO is also known as Devic's syndrome or Devic's disease—and the NMO spectrum disorders are severe idiopathic, inflammatory disorders of the central nervous system (CNS) with a remarkable predilection for the optic nerves and spinal cord. NMO is believed to be a monophasic syndrome characterized by bilateral optic neuritis (ON) and myelitis in rapid succession. The chapter also explores the NMO-IgG marker and how it has changed the clinical diagnosis of NMO, expanded the spectrum of disorders related to NMO, and is helping unravel the pathogenesis of this disorder. NMO usually manifests in adulthood, predominately in late middle age. The cardinal features of ON and myelitis are essential for the diagnosis of NMO. NMO-IgG are a disease biomarker found in the serum of NMO patients. The prognostic value of NMO-IgG is evaluated using both cross-sectional and longitudinal studies, although the data are limited. NMO-IgG are highly specific for the diagnosis of NMO and related disorders and are useful in predicting the recurrence risk of myelitis and/or ON in patients with limited or inaugural presentations of NMO. Discovery of the target antigen, aquaporin-4 (AQP4), promises to elucidate the pathogenesis of NMO. No clinical trial has addressed treatment of NMO exacerbations or relapse prevention exclusively. For long-term treatment, immunosuppressive therapy is recommended.