Objectives: Routine Outcome Monitoring (ROM) is an important quality tool for measuring outcome of treatment in health care. The objective of this article is to summarize the evidence base that supports the provision of feedback on ROM results to (mental) health care professionals and patients. Also, some relevant theoretical aspects are considered. Methods: Literature study (Pubmed, Medline, PsychINFO, Embase Psychiatry, 1975-2009) concerning randomized controlled trials (RTC's) of ROM and feedback on physical or mental health status of patients of all ages. Main search terms were routine outcome monitoring/measurement, feedback, health status measurement, patient reported outcome measures. Results: Included were 52 RCT's concerning ROM and feedback with adult or older patients: of these seven RCT's were exclusively focused on physical health and 45 RCT's (also) on the mental health of the patient, although not always in a mental health care setting or as primary outcome measure. There appears to be a positive impact of ROM on diagnosis and monitoring of treatment, and on communication between patient and therapist. Other results were less clear. There were no published RCT's on this topic with children or adolescents. Conclusions: ROM appears especially effective for the monitoring of patients who are not doing well in therapy. Further research into this topic and the clinical-and cost-effectiveness of ROM is recommended, especially in mental health care for both adults and children. Also, more theory-driven research is needed with relevant conceptualizations such as Feedback Intervention Theory, Therapeutic Assessment.