This chapter considers the structural development of the brain, both prenatally and postnatally. It also considers the nature of developmental change, including evidence for critical\/sensitive periods and innate knowledge. One common idea is that the genetic code provides a blueprint for the structure of the brain. At some gross level, this must be true: all human brains are similar to each other, but differ from the brains of other species. However, a blueprint can\u2019t specify brain structure at a fine level (and the brains of \u201cidentical\u201d twins are not identical). Brain imaging studies reveal that both age of acquisition and level of proficiency determine the neural substrates of second language processing in adults. Heritability estimates can also be applied to structural and functional brain differences as well as to cognitive and behavioral traits. However, genetic influences (measured by heritability) and environmental ones will typically work in combination. For example, gene X environment interactions occur when susceptibility to a trait depends on a particular combination of a gene and environment. <\/p>\n\n\n\n