AbstractNeuronal migration disorders are a category of developmental brain disorders leading to cortical dysplasia. This group of disorders is characterized by defective movement of neurons from the place of origin along the lining of the lateral ventricle, to the eventual place of residence in the correct laminar position within the cerebral cortex. As a result of defective migration, affected individuals typically display mental retardation and epilepsy. Although patients with the more severe forms of these disorders often present during infancy, patients may present at any age from newborn to adulthood. The migration defect may be generalized or focal, and may be disturbed at any of several stages, leading to several distinct radiographical and clinical presentations. The human phenotypes suggests that there are at least four distinct and clinically‐important steps in cortical neuronal migration, and the identification of the responsible genes suggests that multiple cellular processes are critical for correct neuronal positioning. MRDD Research Reviews 2001;7:167–171. © 2001 Wiley‐Liss, Inc.