Neuronal migration disorders are a group of malformations of the brain which primarily affect development of the cerebral cortex. The best known of these is lissencephaly (smooth brain). Most types result from incomplete neuronal migration to the cortex during the third and fourth months of gestation. In this review, we describe and illustrate the different types of neuronal migration disorders. We also review the many different genetic syndromes associated with neuronal migration disorders. Over 25 syndromes with lissencephaly or other neuronal migration disorders have been described. Among them are syndromes with several different patterns of inheritance including chromosomal or new mutation autosomal dominant, autosomal recessive, X-linked and unknown. Genetic counseling thus differs greatly between syndromes. The genes responsible for several of the lissencephaly syndromes have been mapped. X-linked lissencephaly has tentatively been mapped to chromosome Xq22 based on observation of a single X-autosomal translocation in a girl. Both Miller-Dieker syndrome and isolated lissencephaly sequence (in many patients) were mapped to chromosome 17p13.3 by detection of deletions and other structural chromosome rearrangements. Fukuyama congenital muscular dystrophy was mapped to chromosome 9q31-33 by homozygosity mapping.