This paper advocates for process research as a valid source of evidence in clinical psychology, research that focuses on why and how therapy works, both across the course of treatment and in the minutiae of interactions between therapist and client. Process research is consistent with the aims of the scientist-practitioner model, supporting the provision of practical and realistic guidance to clinicians. Specific examples of methods are provided, including the analysis of mechanisms of change, patient-focused research, conversational analysis and interpersonal process recall.