Encephalitis refers to acute inflammatory process affecting the brain. AES may present as encephalitis, meningoencephalitis or meningitis and may be caused by viruses, bacteria, mycobacteria, rickettsia and rarely by toxoplasma. Encephalitis is an inflammation of the brain tissue. The most common cause is viral infections. In rare cases, it can be caused by bacteria or even fungi. Viral infections are most common and important cause of encephalitis. JE and Dengue are more prevalent in South East Asia1.Seasonal outbreaks of acute encephalitis syndrome (AES) occur with striking regularity in India and lead to substantial mortality. Several viruses, endemic in many parts of India, account for AES. Although Japanese encephalitis virus (JEV) is a key aetiological agent for AES in India, and has attracted countrywide attention, this is a systematic review of published studies to understand the altering backdrop of AES in India.