This paper is an overview of hospital utilization review in Spain. Most of the hospital utilization studies have used the Appropriateness Evaluation Protocol as the review instrument. The studies, mainly retrospective, started in the late 1980s and used different adaptations of the medical-surgical version of the protocol. The level of inappropriate use detected ranges between 2.1 and 44.8% for admissions, and from 15 to 43.9% for inappropriate days of stay. The variability in the detected degree of inappropriateness may be due to the differences in the review instrument, the use of the override option, the patients included in the study, or the setting.The determinants of inappropriate hospitalization in Spain are mainly related to access to the different levels of care and to the conservative attitude of the physicians. Future utilization review in Spain should be followed by the development of specific interventions to correct the current pattern of overutilization.