In 2013, the International Encephalitis Consortium proposed clinical criteria for acute encephalitis consisting of 24 hours of altered level of consciousness, lethargy, or personality change and at least three additional supportive features. Although viruses are the most common cause of acute encephalitis, not all encephalitides are acute, viral, or even infectious. Chronic encephalitis can be pathologically similar to acute encephalitis, but the causative agents and clinical manifestations vary. Management of encephalitis is largely supportive; however, for many common encephalitides primary preventive approaches exist. This module reviews the epidemiology, manifestations, diagnosis, management, and prevention of various encephalitides, including herpes family encephalitis (herpes simplex virus, varicella-zoster virus, cytomegalovirus, human herpesvirus 6), arbovirus encephalitis (West Nile virus, eastern equine encephalitis virus, tick-borne encephalitis virus, Japanese encephalitis virus), other encephalitides associated with viruses (influenza virus, human immunodeficiency virus, John Cunningham virus, rabies virus), encephalitides associated with bacteria (Mycoplasma pneumonia, Listeria monocytogenes), and autoimmune encephalitis (acute disseminated encephalomyelitis, paraneoplastic and other autoimmune encephalitides, immune reconstitution inflammatory syndrome). Tables include the International Encephalitis Consortium’s supportive feature of encephalitis, differential diagnosis for magnetic resonance imaging (MRI) findings in the patient with suspected encephalitis, diagnostic considerations for triaging workup of infection-associated encephalitis, differential diagnosis of arboviral infections by location of travel or residence, and preventive strategies for select infectious encephalitis. Figures include MRIs showing patients with herpes simplex encephalitis, varicella-zoster virus, eastern equine encephalitis, HIV, Listeria monocytogenes, acute disseminated encephalomyelitis, acute hemorrhagic leukoencephalitis, and immune reconstitution inflammatory syndrome. This module contains 8 highly rendered figures, 5 tables, 78 references, and 5 MCQs.