Psychological treatments, both for youth and adults, are not always effective. Measurement-based care (MBC) could help to improve the effectiveness. MBC is the routine administration of measures, clinicians’ review of the feedback from these measures, clinicians’ discussion of the feedback with their clients, and collaborative evaluation of the treatment plan. MBC could be the shared stethoscope of the clinician and client in mental health care, as it amplifies the “internal sounds of the mind of the client”, and helps clinicians and clients to listen and interpret the feedback together. However, little is known about how MBC works and the actual use by clinicians is low. The aim of this dissertation was to contribute to both the knowledge and practice base for MBC. In the first part of the dissertation, two studies concerning the preconditions of MBC in youth mental health care are described. We concluded that the Evidence-Based Practice Attitude Scale (EBPAS) is useful for determining the extent to which clinicians have a positive attitude toward evidence-based interventions such as MBC. The Treatment Support Measure (TSM) was found to be useful in providing feedback with concrete tips to improve treatment. In the second part of the dissertation, the results of our efforts to investigate potentially effective components of MBC in youth mental health care and to implement MBC in adult general mental health care are described. No firm conclusions could be reached and implementing and sustaining MBC in real world mental health care proved to be very complex. Therefore, we have provided suggestions at the level of the organization, clinician, and client for possibly improving the implementation and use of MBC. In conclusion, no definitive answer can yet be given as to whether or not MBC should be used. We have indicated future research directions for both theory and practice to improve knowledge about and implementation of MBC. We expect that this will clarify that and how MBC can improve treatments for both youth and adults. As a result, MBC could become the shared stethoscope of clinicians and clients in mental health care.