Attempts to regulate service delivery in line with results from randomized trials have been vigorously debated. In this paper, results from practice-based studies using the CORE System illustrate the potential to enrich knowledge about the actual outcome of psychological therapy in routine care. These studies also provide data for important questions in psychotherapy research, like orientation differences, the importance of the therapist factor, number of sessions needed for clinical effect, and the alliance-outcome question. Obstacles and challenges in making such studies are illustrated. In conclusion, arguments are put forward for introducing a common measurement system that strikes a balance between clinicians' questions and the need for comparable data, and that encompasses the complexities of patients' reasons for seeking psychological help.