This paper provides a conceptual overview of recent research on the developmental psychobiology of childhood stress. We propose that the construct of temperament can be regarded as an organizing principle in children's responses to environmental adversity, and we review three physiologic "windows" through which such responses can be usefully examined. It is concluded that individual differences between children in their psychobiologic reactivity to stressors may constitute an important and novel point of departure for future studies of stress and childhood morbidity.