Understanding how the cortex develops has gained very important new data thanks to both experimental and clinical studies. Experimental studies have shown that: --neurons are generated in the ventricular zone by asymmetric mitoses; --the first cortical region to differentiate is the so-called pre-plate that plays a major role in the control of neuroblasts migration; --neuroblasts arise according to an inside-outside gradient; they migrate either along the processes of radial cells or according to a new type of non radial migration; --all the cortical neurons are not generated by the dorsal telencephalon; some of these neurons derive from the basal telencephalon; --neuroblasts acquire their specificity during their way to the cortical plate. There are several genetic syndromes leading to a malformation of the cortex. Classic lissencephaly is essentially due to mutations of the LIS1 or the DCX genes. These genes code for proteins that are involved in cytoskeleton functions. Reelin is responsible for a human syndrome associating pachygyria and cerebellar hypotrophy. Subventricular heterotopia can be X- inherited and are due to a mutation of the gene coding for filamin 1.