The Polyvagal Theory, introduced by Stephen W. Porges, offers a new perspective on the relationship between autonomic function and behavior. It emphasizes the autonomic nervous system as a "system" and identifies neural circuits involved in regulating autonomic state. The theory interprets autonomic reactivity as adaptive within the context of vertebrate autonomic nervous system phylogeny. The paper aims to provide an explicit statement of the theory and introduce the features of a polyvagal perspective. This perspective highlights the importance of phylogenetic changes in neural structures regulating the autonomic nervous system and how these changes provide insights into the adaptive function and neural regulation of the two vagal systems. The theory challenges traditional peripheral models by integrating neurophysiological mechanisms and phylogenetic shifts in neural regulation, leading to different questions, paradigms, explanations, and conclusions about autonomic function in biobehavioral processes. The Polyvagal Theory also introduces the concept of the "vagal brake," which is a unique feature of mammalian autonomic regulation, and discusses the Social Engagement System, a phylogenetically integrated system that links behavioral and autonomic components. The theory provides a neurobiological model linking difficulties in spontaneous social behavior to facial expressivity and visceral state regulation, and suggests that social behaviors can regulate physiological activity and vice versa.The Polyvagal Theory, introduced by Stephen W. Porges, offers a new perspective on the relationship between autonomic function and behavior. It emphasizes the autonomic nervous system as a "system" and identifies neural circuits involved in regulating autonomic state. The theory interprets autonomic reactivity as adaptive within the context of vertebrate autonomic nervous system phylogeny. The paper aims to provide an explicit statement of the theory and introduce the features of a polyvagal perspective. This perspective highlights the importance of phylogenetic changes in neural structures regulating the autonomic nervous system and how these changes provide insights into the adaptive function and neural regulation of the two vagal systems. The theory challenges traditional peripheral models by integrating neurophysiological mechanisms and phylogenetic shifts in neural regulation, leading to different questions, paradigms, explanations, and conclusions about autonomic function in biobehavioral processes. The Polyvagal Theory also introduces the concept of the "vagal brake," which is a unique feature of mammalian autonomic regulation, and discusses the Social Engagement System, a phylogenetically integrated system that links behavioral and autonomic components. The theory provides a neurobiological model linking difficulties in spontaneous social behavior to facial expressivity and visceral state regulation, and suggests that social behaviors can regulate physiological activity and vice versa.