Within the depth-psychology corpus, the parasympathetic nervous system occupies a pivotal conceptual position — not as mere physiological counterweight to sympathetic arousal, but as the neurobiological substrate of safety, restoration, and relational engagement. The dominant theoretical framework shaping its treatment is Stephen Porges’s Polyvagal Theory, which fractures the classical two-branch autonomic model by distinguishing two phylogenetically and functionally distinct parasympathetic pathways: the ancient dorsal vagal circuit, mediating immobilization and shutdown under life-threat, and the evolutionarily recent ventral vagal circuit, enabling social engagement and co-regulation. This distinction has proven generative across trauma therapy (van der Kolk, Ogden, Rothschild), somatic approaches (Levine, Heller), and clinical adaptation (Dana). A second strand, represented by Heller and Fogel, preserves the classical homeostatic framing — the parasympathetic as the restorative counterpart to sympathetic mobilization, governing heart rate reduction, digestion, and immune function. A third strand, found in Damasio and Porges’s earlier theoretical writings, situates both branches within the broader somatic-marker and affect-regulation frameworks. Key tensions persist between the two-branch and three-circuit models, and between the parasympathetic as passive brake versus active architect of mammalian social life.