Hindbrain herniation is only one component of the Chiari malformations. These malformations often have other associated intracranial anomalies and malformations of the vertebral column. Depending on embryologic timing, herniation of the cerebellar vermis or tonsils occurs. Additionally, the herniated hindbrain may include the medulla oblongata and fourth ventricle. Currently, no single theory explains all of the malformations seen in the Chiari I and II malformations. These pathologic derailments seem to result from a heterogeneous spectrum of ontogenetic errors and pathological mechanisms, which share some common phenotypical presentations. In this chapter, the theories pertinent to the embryology and pathophysiology of Chiari I and II malformations and their associated anomalies are discussed.