The mechanisms that govern the development of the neural circuitry are still largely unknown. It has not yet been clearly established to what extent early cortical differentiation depends on environmental or genetic factors. The first part of this book reports on findings of transplantation studies, performed in newborn recipient rats, in which these developmental issues are addressed. In humans, the cerebral cortex can be affected by a variety of diseases, hence several experimental studies were undertaken to determine to what extent transplantation of cortical neurons could be proven a useful treatment for cerebral cortical damage. The second part deals with the functional aspects of transplantations in adult recipient rats.