Normal central nervous system development is dependent on extensive cell migration. Cells born in the proliferative ventricular zone migrate radially along specialized glial processes to their final locations. In contrast, most inhibitory interneurons found in the adult mammalian cerebral cortex and some other structures migrate along a nonradial pathway and on substrates only recently defined. Defects in radial cell migration have been implicated in several distinct human syndromes in which patients often present with epilepsy and mental retardation and have characteristic cerebral abnormalities. The identification of several genes responsible for human neural cell migration defects has led to a better understanding of the cellular and molecular interactions necessary for normal migration and the pathogenesis of these disorders. The prototypic cell migration disorder in humans is type I lissencephaly. Although type 1 lissencephaly is clearly a defect in radial cell migration, recent data from two model systems (Lis1 and ARX mutant mice) indicate that a defect in non-radial cell migration also exists. Thus, the result of a LIS1 mutation appears to have broader implications than a radial cell migration defect alone. Furthermore, it is likely that the observed defect in non-radial cell migration contributes to the clinical phenotype observed in these patients. Herein we discuss the role of normal non-radial cell migration in cortical development, as well as how perturbations in both radial and nonradial migration result in developmental anomalies.