In the last decade, cardiac vagal tone has emerged as a psychophysiological marker of manyaspects of behavioral functioning in both children and adults. Research efforts during this timehave produced an extensive list of vagal tone correlates that includes temperamental variables aswell as both anxious/internalizing and disruptive/externalizing behaviors. This potentiallyconfusing state of affairs is compounded by developmental shifts in vagal tone–behaviorrelations that to date have not been elucidated. In this paper, the vagal tone literature is reviewed,and discrepancies, including the lack of specificity of vagal tone as a psychophysiological marker,are clarified. Such clarification requires that we (a) view vagal tone–behavior relations indevelopmental context, (b) juxtapose vagal tone–behavior relations in typical and atypicalsamples, and (c) consider the parasympathetic underpinnings of vagal tone as but one componentin a broader model of autonomic nervous system functioning. Such a model is provided bycombining Gray's motivational theory with Porges's polyvagal theory. Togetherthese models account for behavioral and emotional differences in a diverse range of psychologicaldisorders that are not differentiated by either model alone. Moreover, use of the integrated modeloffers a theory-driven approach to the study of autonomic nervous system–behaviorrelations.