Intractable epilepsies and partial epilepsies, which make up a great majority of epileptic disorders, are not better recognized and their etiologies unveiled with the help of the new imaging techniques. The development of magnetic resonance imaging (MRI) permits the accurate diagnosis while the patients are alive of the neuronal migration disorders (NMD), which constitute an important group of intractable epilepsies. Previously, NMD cases were described by neuropathologists from autopsy materials, and many of these developmental disorders were not considered compatible with prolonged survival. Cerebral malformations due to neuronal migration anomalies are described in association with motor and mental retardation, learning disabilities, microcephaly, dysmorphic features and epilepsy. Neuronal migration takes place in all parts of the central nervous system (CNS) during the shaping process of the CNS; it actually includes both the central and peripheral nervous systems. However, in common usage the meaning of "neuronal migration disorders" is restricted to the neocortex.