The present work presents the polivagal theory as a bio-behavioral model of emotional and interpersonal behavior, aiming to identify its theoretical contributions to the study of psychotherapy regarding client´s clinical status, therapeutic process as well as change. A review of the model shows a reconceptualization of interactions among autononomic nervous system and emotionality within interpersonal context, allowing for identifying normal and pathological activation of defensive behavioral strategies (figh-or-flight and immobilization), as well as their inhibition through the social engagement system. Empirical research shows evidence in favor of hypotheses derived from the model in normal and clinical populations. In clinical contexts, the social engagement system is of particular interest as it allows the emergence of interpersonal trustworthy behavior, being a cornerstone in psychotherapy due to its potential diagnostic value for clients, the development of the therapeutic relationships, as well as beneficial outcomes. Lastly, a nascent body of psychotherapy studies assessing the social engagement system (through high frequency heart rate variability-) and its relation with the therapeutic process (i.e. working alliance) and outcomes (i.e. symptom level) are identified. The evidence supports the value of further developing this interdisciplinary line of research and, eventually, incorporating these measurements into the clinical assessment arsenal.