Encephalitis is a term which has been applied to a wide variety of diseases of the central nervous system. The clinician uses this term to designate an obscure, diffuse disease of the brain, whether of proved inflammatory nature or not. The pathologist employs it to denote any condition in which there is objective evidence of cerebral inflammation. Three large subdivisions have been made on an etiologic, pathologic and clinical basis. These are the toxic or hemorrhagic, the bacterial or suppurative and the nonhemorrhagic nonsuppurative types. The toxic form may be recognized with some certainty clinically and pathologically, because it is often possible to link intoxication with arsenic, guanidine, lead and other poisons with the onset of clinical symptoms referable to the central nervous system and because hemorrhages and other changes in the brains are found at autopsy. The bacterial form may be established by cultivation of micro-organisms from the injured