Acute disseminated encephalomyelitis (ADEM) is an inflammatory demyelinating disorder mediated by the immune system, affecting the central [Page 23]nervous system (CNS). The International Pediatric Multiple Sclerosis Study Group defines ADEM as an illness presenting with multiple neurologic symptoms and encephalopathy. ADEM is almost exclusively a pediatric disease with the average age at presentation between 5 and 8 years. ADEM affects both males and females, with a slight male predominance, overall affecting 0.4 per 100,000 children per year. Although the neurologic prognosis of ADEM is favorable, research since the early 2000s suggests that the implications of ADEM on neurodevelopment and cognition may be underrecognized. This entry addresses the clinical and radiologic features, pathogenesis, common differential diagnoses, treatment, and prognosis of ADEM.