Drug utilization review (DUR) system has been defined as "structured, ongoing initiatives that interpret patterns of drug use in relation to predetermined criteria, and attempt to prevent or minimize inappropriate prescribing." This paper introduces the concept, purpose, and effective application of OUR in Korea. OUR can be classified as retrospective OUR, prospective OUR, and concurrent OUR based on the time direction of applying OUR. OUR can also be classified as quantitative DUR defined by retrospective OUR using databases including previously prescribed medicines, and qualitative DUR defined by OUR reflecting patient`s clinical condition. We described the history of developing OUR in the United States and the Europe. Finally current sta...