Human interaction, including psychotherapy, is systematically responsive; therapists' and clients' behavior is influenced by emerging context, including perceptions of each other's characteristics and behavior. Feedback and mutual influence occur on a wide range of time scales, including treatment assignment, strategy, and tactics, -and even within the delivery of interventions. Consequently, research that assumes linear relations among psychotherapeutic variables may not be trustworthy. The concept of responsiveness helps show how client characteristics, therapist characteristics, and process components may be important in psychotherapy despite a lack of linear relations to outcomes. Research strategies that incorporate responsiveness include the use of evaluative measures, systems approaches, and qualitative and narrative approaches.