Since the discovery of aquaporin 4-IgG, a sensitive and highly specific biomarker of neuromyelitis optica (NMO), a wide range of syndromes have been recognized as being associated with this condition. This observation has led to new proposed terminology for the entire disorder, NMO spectrum disorders (NMOSD). The discovery of a pathogenic autoantibody and its target antigen has also facilitated basic research into the immunopathogenesis of the disease. Key advances include establishment of passive transfer animal models demonstrating the pathogenic potential of the autoantibody and confirming an important role of complement suggested by immunopathology of NMO brain lesions and of B-cell subsets, plasmablasts in particular. These discoveries have led to phase 1 clinical trials of targeted immunotherapy with potential for improved efficacy and less toxicity than current empiric immunosuppressant medications used to treat NMOSD. Randomized clinical trials are beginning to assess the efficacy and safety of a variety of immunotherapies in NMOSD. Therapeutic options are likely to increase, and improved outcomes in NMOSD patients are anticipated.