The Polyvagal Theory, introduced by Stephen W. Porges, offers a new perspective on autonomic function in relation to behavior. It emphasizes the autonomic nervous system as a "system," identifies neural circuits involved in autonomic regulation, and interprets autonomic reactivity as adaptive within the context of vertebrate phylogeny. The theory challenges traditional models by integrating neurophysiological mechanisms and phylogenetic shifts in neural regulation, leading to new questions, paradigms, and explanations regarding autonomic function in biobehavioral processes. The theory highlights the importance of phylogenetic changes in neural structures regulating the autonomic nervous system and their implications for adaptive function and neural regulation of the two vagal systems. The Polyvagal Theory provides a framework for understanding the adaptive nature of physiological states, emphasizing how different physiological states support different classes of behavior. It also highlights the functional and structural links between neural control of facial muscles and smooth muscles of the viscera. The theory proposes a mechanism called neuroception, which evaluates risk and modulates vagal output via higher brain structures. It also emphasizes the neurophysiological and neuroanatomical distinction between two branches of the vagus, each supporting different adaptive behavioral strategies. The theory articulates three phylogenetic stages of the development of the vertebrate autonomic nervous system, each associated with distinct autonomic subsystems or circuits. These circuits are organized in a phylogenetically-determined hierarchy, consistent with the Jacksonian principle of dissolution. The theory links the evolution of the autonomic nervous system to affective experience, emotional expression, facial gestures, vocal communication, and contingent social behavior. The vagal brake, a unique feature of mammals, functions as an active vagal brake that can rapidly mobilize or calm an individual. The myelinated vagus actively inhibits the sympathetic nervous system's influences on the heart and dampens the HPA axis activity. The vagal brake provides a neural mechanism to rapidly change visceral state by slowing or speeding heart rate. The amplitude of RSA provides an assessment of the state of the vagal brake, and the Polyvagal Theory frames a research agenda by proposing that the vagal brake is related to behavioral and psychological processes along a continuum from prosocial-affiliative interactions to adaptive fight/flight behaviors. RSA is a naturally occurring rhythm in the heart rate pattern at approximately the frequency of spontaneous breathing. The amplitude of RSA can be quantified to provide a sensitive index of the impact of the myelinated vagus on the heart. RSA is correlated with emotional expressivity, social engagement, and emotional regulation. Lower RSA is associated with difficulties in social and emotional regulation and is linked to psychiatric disorders. Higher RSA is associated with greater self-reported regulatory control and decreased negative emotional arousal in response to stressors. RSA level appears to parallel the positive effects of treatment, with increases in RSA being reported only in patients with depression who exhibited a clinically significant response to treatment. Greater RSA suppressionThe Polyvagal Theory, introduced by Stephen W. Porges, offers a new perspective on autonomic function in relation to behavior. It emphasizes the autonomic nervous system as a "system," identifies neural circuits involved in autonomic regulation, and interprets autonomic reactivity as adaptive within the context of vertebrate phylogeny. The theory challenges traditional models by integrating neurophysiological mechanisms and phylogenetic shifts in neural regulation, leading to new questions, paradigms, and explanations regarding autonomic function in biobehavioral processes. The theory highlights the importance of phylogenetic changes in neural structures regulating the autonomic nervous system and their implications for adaptive function and neural regulation of the two vagal systems. The Polyvagal Theory provides a framework for understanding the adaptive nature of physiological states, emphasizing how different physiological states support different classes of behavior. It also highlights the functional and structural links between neural control of facial muscles and smooth muscles of the viscera. The theory proposes a mechanism called neuroception, which evaluates risk and modulates vagal output via higher brain structures. It also emphasizes the neurophysiological and neuroanatomical distinction between two branches of the vagus, each supporting different adaptive behavioral strategies. The theory articulates three phylogenetic stages of the development of the vertebrate autonomic nervous system, each associated with distinct autonomic subsystems or circuits. These circuits are organized in a phylogenetically-determined hierarchy, consistent with the Jacksonian principle of dissolution. The theory links the evolution of the autonomic nervous system to affective experience, emotional expression, facial gestures, vocal communication, and contingent social behavior. The vagal brake, a unique feature of mammals, functions as an active vagal brake that can rapidly mobilize or calm an individual. The myelinated vagus actively inhibits the sympathetic nervous system's influences on the heart and dampens the HPA axis activity. The vagal brake provides a neural mechanism to rapidly change visceral state by slowing or speeding heart rate. The amplitude of RSA provides an assessment of the state of the vagal brake, and the Polyvagal Theory frames a research agenda by proposing that the vagal brake is related to behavioral and psychological processes along a continuum from prosocial-affiliative interactions to adaptive fight/flight behaviors. RSA is a naturally occurring rhythm in the heart rate pattern at approximately the frequency of spontaneous breathing. The amplitude of RSA can be quantified to provide a sensitive index of the impact of the myelinated vagus on the heart. RSA is correlated with emotional expressivity, social engagement, and emotional regulation. Lower RSA is associated with difficulties in social and emotional regulation and is linked to psychiatric disorders. Higher RSA is associated with greater self-reported regulatory control and decreased negative emotional arousal in response to stressors. RSA level appears to parallel the positive effects of treatment, with increases in RSA being reported only in patients with depression who exhibited a clinically significant response to treatment. Greater RSA suppression