This review describes the prenatal and postnatal development of the human cortex. Neurogenesis, neuronal migration, dendrite maturation, synaptogenesis, and white matter development are discussed. In addition, the concept of "critical" or "sensitive" periods is discussed as well as genetic and environmental influences (Nature-Nurture). The effects of irradiation, alcohol, smoking, and prenatal maternal influenza and stress on brain functions and language performance are reviewed. The periods of plasticity are reviewed for stereoscopic vision, brain lesions, social neglect, and first and second language acquisition. This article illustrates that "critical" or "sensitive" periods exist for those functions for which axonal rewiring across a long distance in the brain is necessary. This is virtually impossible after completion of the developmental competition between different axonal systems. Furthermore, genotyping allows the maximum of structural and functional properties, but the eventual outcome is under the influence of the environment in positive or negative directions. Under dramatic circumstances, environmental factors during the first years of life can be quite decisive for the development of language, social, and other intellectual functions.