Book excerpt: This book for health care practitioners presents the functional importance of the autonomic nervous system (ANS) in the physiological, behavioral, and psychological balance in humans. It clearly introduces Stephen W. Porges' polyvagal theory and presents applications to human health.Older and dualistic concepts of the sympathetic and parasympathetic nervous systems hold that they operate in complementary and opposing ways; here, the reader will discover a new system with three hierarchical levels arranged according to their functions for adaptation, survival, and homeostasis:?the dorsal vagal branch, which is the most ancient, is responsible for the primary parasympathetic functions;?the sympathetic system, which is in charge of energy mobilization, work, fight, or flight;?the ventral vagal branch, which appeared late in the evolution of mammals, and regulates the thoracic organs, head, and neck, which together with other cranial nerves, are responsible for the social engagement system.The polyvagal theory also serves as a central pillar around which a new explanation of human development is proposed, regrouping physiological, psychological, and spiritual dimensions, and shedding new light on psychosomatic phenomena.In addition to giving a clinical semiological presentation, the chapter on heart rate variability shows how it is possible to quantify the performance of the ventral branch of the vagus nerve and its interaction with the sympathetic system. Moreover, the innovative osteopathic techniques proposed in this book constitute a new approach for treating the vagus nerve and the ANS.Although this book was written by an osteopath, it will benefit all therapists of the body, mind, and spirit who search to deepen their comprehension of human functioning.