This chapter presents the definition, epidemiology, pathology, physiology, clinical features, diagnostic tests and principles of management for Rasmussen encephalitis (RE). The histopathological properties of RE are non-specific and are suggestive of chronic multifocal encephalitis. Rasmussen hypothesized a viral etiology, based on the constituents of the immune reaction in the brain such as lymphocyte infiltration and microglial nodules. Magnetic resonance imaging (MRI) of the brain shows progressive hemiatrophy of variable degree in the involved cerebral hemisphere. Fluorodeoxyglucose positron emission tomography (PET) studies showed large areas of hypometabolism which were confined to the affected hemisphere. An early surgery seems to be favored in view of the progressive neurologic devastation and dementia that may directly result from the catastrophic epilepsy. The late complications of the early surgical techniques, including superficial hemosiderosis and hydrocephalus, have been largely overcome by the introduction of disconnective procedures such as functional hemispherectomy and hemispherotomy.