One of the central tenets of developmental psychopathology is the belief that we canlearn more about normal functioning through the study of psychopathology and arrive at a betterunderstanding of pathological conditions through investigations of normal behavior. Advances inknowledge from one area will inform us regarding mechanisms at work in the other. A similarperspective is the driving force behind recent scientific advances in our understanding of certaindevelopmental disorders. In this paper, molecular findings for four developmental disorders arereviewed: Prader–Willi syndrome, fragile X syndrome, Williams syndrome, andlissencephaly. These disorders were chosen for discussion because putative genes for each ofthem have been isolated. The ways in which mutations within these genes disrupt normalcognitive and behavioral functioning are discussed. Although considerable progress has beenachieved in understanding the genetic mechanisms for these illnesses, much more research isneeded to identify the environmental and genetic factors that interact to contribute to theexpression of the more complex behavioral disorders.