Polymicrogyria (PMG) is a complex cortical malformation which has so far defied any mechanistic or genetic explanation. Adopting a broad definition of an abnormally folded or festooned cerebral cortical neuronal ribbon, this review addresses the literature on PMG and the mechanisms of its development, as derived from the neuropathological study of many cases of human PMG, a large proportion in fetal life. This reveals the several processes which appear to be involved in the early stages of formation of polymicrogyric cortex. The most consistent feature of developing PMG is disruption of the brain surface with pial defects, over-migration of cells, thickening and reduplication of the pial collagen layers and increased leptomeningeal vascularity. Evidence from animal models is consistent with our observations and supports the notion that disturbance in the formation of the leptomeninges or loss of their normal signalling functions are potent contributors to cortical malformation. Other mechanisms which may lead to PMG include premature folding of the neuronal band, abnormal fusion of adjacent gyri and laminar necrosis of the developing cortex. The observation of PMG in association with other and better understood forms of brain malformation, such as cobblestone cortex, suggests mechanistic pathways for some forms of PMG. The role of altered physical properties of the thickened leptomeninges in exerting mechanical constraints on the developing cortex is also considered.