Therapist responsiveness denotes that therapists provide therapeutic interventions within an emerging context of client manifestations and moment-by-moment internal and external changes. So far, psychotherapy research on explaining how therapy works falls short of operationalizing the sequence of events constituted by therapist responsiveness. The present special section of Psychotherapy Research addresses this conceptual and methodological gap and proposes six original contributions, using several validated assessment protocols, both from a quantitative and qualitative viewpoint, to study therapist responsiveness in psychotherapy. It aims at providing a rigorous conceptual and methodological basis for studying a core principle of change in psychotherapy for the future.