Neuromyelitis optica of Devic (NMO) is a syndrome which combines transverse myelitis and optic neuritis. For many years it was assumed that transverse myelitis and optic neuritis should be simultaneous, that optic neuritis should be bilateral, and that transverse myelitis must be complete. NMO was considered to be very rare. Through the 20th century there was a debate on whether NMO is a distinct nosologic entity or a form of MS. In 2005 it is now possible to distinguish NMO from MS on the basis of the clinical, imaging, serology and immunopathology profile. This distinction is necessary for the implementation of adequate treatments, as the relapsing form of the illness carries significant morbidity and mortality.