Malformations of cortical development are frequently identified in surgical resections for intractable epilepsy. Among the more frequently identified are cortical dysplasia, pachygyria, and polymicrogyria. The pathogenesis of these common developmental anomalies remains uncertain. Polymicrogyria is particularly vexing because there are multiple described forms (2, 4, and 6 layers) that have been attributed to multiple etiologies (e.g. ischemic, genetic, infectious, and toxic). We reviewed the pathology in 19 cases and performed cortical laminar analysis in 10 of these cases. Our data indicate that a defining feature of polymicrogyriais fusion of the molecular layer and that most often there is a well-defined gray matter-white matter junction. Unexpectedly, the cortical laminae were normally positioned, but there were reduced neuronal populations within these laminae, particularly in the subgranular layers. On the basis of these data, we propose that the categorization of polymicrogyria according to the number of lamina is artificial and should be abandoned, and polymicrogyria should be defined according to the presence or absence of coexisting neuropathological features. Furthermore, our data indicate that polymicrogyria is not a cell migration disorder, rather it should be considered a postmigration malformation of cortical development.