Reelin, synthesized and secreted by Cajal-Retzius (CR) cells in the marginal zone of the cortex, is an extracellular matrix protein important for the development of cortical layers. In reeler mutant mice lacking Reelin, there are severe malformations of neocortical and hippocampal lamination. It has been assumed that Reelin acts as a stop signal for migrating neurons. Here we show, by using the dentate gyrus as a model in in vivo studies and in vitro assays, that Reelin exerts its effects, at least in part, by acting on the radial glial scaffold required for neuronal migration. Migration defects of dentate granule cells, reminiscent of those seen in reeler mutants, are observed in tissue from patients with temporal lobe epilepsy (TLE). The extent of granule cell dispersion in TLE was found to be inversely correlated with the number of reelin mRNA synthesizing CR cells and reelin mRNA expression as revealed in quantitative RT-PCR studies. These findings show that the Reelin signaling pathway is essential for the correct positioning of human hippocampal neurons and that a Reelin deficiency is involved in the pathological changes associated with epilepsy.