One of the central tenets of developmental psychopathology is the belief that we can learn more about normal functioning through the study of psychopathology and arrive at a better understanding of pathological conditions through investigations of normal behavior. Advances in knowledge from one area will inform us regarding mechanisms at work in the other. A similar perspective is the driving force behind recent scientific advances in our understanding of certain developmental disorders. In this paper, molecular findings for four developmental disorders are reviewed: Prader–Willi syndrome, fragile X syndrome, Williams syndrome, and lissencephaly. These disorders were chosen for discussion because putative genes for each of them have been isolated. The ways in which mutations within these genes disrupt normal cognitive and behavioral functioning are discussed. Although considerable progress has been achieved in understanding the genetic mechanisms for these illnesses, much more research is needed to identify the environmental and genetic factors that interact to contribute to the expression of the more complex behavioral disorders.