The present paper addresses research applications to individual psychotherapy of interpersonal inventories derived from contemporary interpersonal theories of personality, psychopathology, and psychotherapy. It illustrates how interpersonal theory and circle inventories can form an integral part of the conceptualization and assessment of important constructs that have derived from various psychotherapy theories. It addresses the key relationship concepts of therapeutic alliance, social reinforcement, resistance and countercontrol, transference, and countertransference. The paper then analyzes the issues of the effectiveness of psychotherapy interventions, interventive stages, psychotherapy outcome, therapist-patient matching diagnosis of personality disorders, and parallel process in psychotherapy and supervision.