Are we the product of our genes or our experiences? The nature-nurture debate has been at the forefront of discussions about developmental processes for centuries. Over the past few years, however, significant technical and methodological advances both in genetics and in the prospective longitudinal study of children (and certain animal species) have generated a set of new findings that may well render many of the previous nature-nurture arguments largely moot. Scientists can now identify and even characterize the expression not only of individual genes in specific tissues but indeed across the entire genome. It is now clear that whereas the DNA sequences that make up any one gene are essentially fixed throughout an individual’s lifetime (except for mutations), the expression of that gene can be modified by a variety of factors, some of which are external to the organism. It is also now clear that certain experiences, particularly at certain times during development, can effectively “get under the skin,” i.e., they can result not only in long-term changes in behavior but also in neuroendocrine, neurochemical, and immunological functioning, strongly suggesting that development is not the exclusive product of either nature or nurture but rather of their interaction, i.e., gene-environment interplay. This symposium will present four sets of findings from prospective longitudinal studies of children and rhesus monkeys that highlight such gene-environment interplay.