A systematic review of the methods used to assess appropriateness of acute bed use and the evidence on the scale of inappropriate use in different patient groups is presented. Issues of generalisability of the findings are also addressed. Criteria based tools are the accepted way of measuring inappropriate days of stay and admissions, although opinion based classification is very common. While a number of tools exist, few have been adequately tested for reliability and validity. The Appropriateness Evaluation Protocol (AEP) is the most commonly used tool, and has been tested more widely. It appears to be both reliable and valid. An estimated 29% of admissions to acute psychiatric may be inappropriate. Regarding days of care after admission, between 24 and 58% of stays were not judged to be appropriate for continued stay on an acute ward. The need for continued acute psychiatric care may become lower as patients experience continued stay in the acute setting. A lack of housing and community support was the most commonly cited reason preventing discharge. Rates of inappropriate use appear to be higher for older patients than for the general population. Wide variation in rates of inappropriate days of stay was found, but it may be safe to assume that inappropriate use is greater than 20% across a wide variety of settings. Reasons for older patients to remain in an acute hospital bed after medically necessary are typically moderate nursing care needs (i.e. long-term care). The estimates of inappropriate use in other groups was found to be highly variable. Before definitive conclusions on the inappropriate use of acute beds can be made, future research needs to take into account the methodological problems discussed here.