More than a century has passed since the first description of neuromyelitis optica (NMO) or Devic's disease. The relation between NMO and multiple sclerosis (MS) had long been debated, but the discovery of anti-aquaporin-4(AQP4) antibody, an NMO-specific autoantibody has accelerated clinical and experimental research of NMO, and contributed to estabilishing NMO spectrum disorder(NMOSD), a wider disease spectrum than a prototypic opticospinal phenotype and a new disease concept: autoimmune astrocytopathic disease. Clinical, MRI and laboratory findings and therapeutic response in NMOSD are different from those in MS. On the other hand, anti-AQP4 antibody-seronegative NMOSD has some distinct features from seropositive cases, and anti-myelin oligodendrocyte glycoprotein(MOG) antibody is detected in a fraction of cases of seronegative NMOSD. Unlike anti-AQP4 antibody-associated NMOSD, anti-MOG antibody-seropositive NMOSD may be a demyelinating desease. In this review, we provide an overview of how the concept of NMOSD has evolved in association with accumulated scientific evidences.