Acute transverse myelitis (ATM) is an inflammatory demyelinating disorder that affects the spinal cord focally resulting in motor sensory and autonomic dysfunction. Establishing the diagnosis of ATM is not as difficult as determining the possible etiology. There is a difference in the perception of ATM seen in the West as compared to developing countries. In the West multiple sclerosis (MS) is the most common inflammatory disorder of the central nervous system. An attack of ATM may be the beginning of MS. However, this may not be the case in developing countries where MS is uncommon. Most often transverse myelitis is monophasic and at best represents a site-restricted form of acute disseminated encephalomyelitis (ADEM). Traditionally the combination of optic neuritis and ATM, occurring as a monophasic illness would have been called as neuromyelitis optica (NMO). Changing concepts in the definition of NMO and the discovery of a biomarker, neuromyelitis optica immunoglobulin (NMO_IgG), has changed the way relapsing autoimmune disorders are being perceived currently. A variety of idiopathic inflammatory disorders such as Japanese form of optic spinal MS, recurrent myelitis, and recurrent optic neuritis have been brought under the umbrella of neuromyelitis spectrum disorders because of the association with NMO-IgG. Complete transverse myelitis accompanied by longitudinally extensive transverse myelitis which is seronegative for this biomarker has also been reported from several countries including Japan, Australia, and India. Thus, ATM is a heterogeneous disorder with a varied clinical spectrum, etiology, and outcome.