A classification of neuronal migration disorders based on the gross morphology of the disorder with subclassifications according to the cause of the underlying disorder as far as that is known is presented. Emphasis is on disorders of the neocortex. The classical entities classical lissencephaly, cobblestone lissencephaly, polymicrogyria, neuronal heteropia within the white matter and leptomeningeal glial and glioneuronal heterotopia are described. Recent findings on the genetics of neuronal migration disorders are discussed. More subtle structural abnormalities such as cortical dysgenesis or dysplasia remain still more controversial as separate entities.