The previous volumes in this series have dealt with the mature cerebral cortex. In those volumes many of the structurally and physiologically distinct areas of the cerebral cortex, their connections, the various types of neurons and neuroglial cells they contain, and the functions of those cells have been considered. In the present volume the contributions focus on the development of the neocortex and hippocampus. Chapters in this volume describe how the neurons migrate in the cortex to attain their ultimate positions, and emphasize the role played by the preexisting pallium or primordial plexiform layer of the cerebral vesicle in the development of the cerebral cortex. The primordial plexiform layer becomes split by the invasion of neurons that will form the cortical plate, and mutants in which the neuronal migration is abnormal provide valuable information about the role of the radial glial cells in this migration. It is also made clear that although the mechanics of development in the hippocampus are similar to those in the neocortex, the development of the hippocampus involves some unique features. For example, neuronal proliferation in the dentate gyrus continues well into postnatal life.