This chapter emphasises the importance of the intersubjective physiological field between the therapist and the client and the significance of the therapist’s physiological state during the therapy session. The authors discuss the role of three pathways of the autonomic nervous system (ventral vagal, sympathetic, and dorsal vagal) and related autonomic states, based on Porges’ polyvagal theory. They also review the research into interpersonal autonomic physiology and relate this to the concepts of emotional contagion, mimicry, and mirror neurons. The authors apply polyvagal theory and interpersonal physiology research to psychotherapeutic work. The physiological states of the therapist and the client are interconnected, and they tend to synchronise. The therapist’s autonomic nervous state profoundly influences the therapeutic work. The authors discuss the positive and negative aspects of physiological synchrony in the therapeutic relationship, and the importance of therapist self-regulation and regulation of the client. The authors also emphasise the importance of the therapist’s mindful awareness of their own internal body signals during the therapy session for the regulation of their own and the client’s physiological states.