The development of magnetic resonance imaging (MRI) has led to an improved understanding of cerebellar herniation. The original studies of Hans Chiari dealt with four types of cerebellar malformation in these autopsy studies. These four types represent completely different pathologies. The Chiari III malformation and Chiari IV malformation (cerebellar agenesis) are extremely rare and are unlikely to be seen in the adult population. Chiari I and Chiari II are now frequently cared for in adult settings. Chiari I malformation is a crowding of the posterior fossa with tissue primarily derived from the inferior-most part of the cerebellar hemispheres, generally called the cerebellar tonsils. The radiographic appearance of the Chiari II malformation is much improved and the incidence of hydrocephalus that needs to be shunted falls to a minority of patients. The most likely situation in which Chiari I malformation and pseudotumor will coexist occurs in the context of morbid obesity.