This paper describes the development and testing of a European version of the Appropriateness Evaluation Protocol (AEP). It stemmed from the original U.S. version and the multiple adaptations and modifications made previously and separately by researchers in European countries. The group was particularly concerned with developing a common list of reasons for inappropriate admissions and days of stay, since the principal goal was to enable an understanding of inappropriate hospital use and potential solutions within local health and social care systems. Developing a common EU-AEP included several steps. First, each national instrument was translated from the national language to English. These back translations were compared with each other and with the US-AEP. A working group analyzed the content of the lists of reasons published in the literature and proposed a novel conceptual approach. On the basis of workshop discussions, a draft of a common European version was circulated to each participant for agreement. In the EU-AEP, the clinical criteria for the appropriateness of admission include 10 related to patient condition and five to clinical services. The criteria for the appropriateness of days of care include 10 covering medical services, six for life support/nursing services, and eight related to patient condition. The proposed core list of reasons of inappropriateness distinguish clearly between two concepts: a) the level of care required by the patient; and b) the reason why this level of care was not used. The first list would thus refer to the nature of resources and facilities required, while the second would focus more on the efficient organization of those resources. A validated European tool to assess inappropriate hospital admissions and hospital days of stay and their causes might be used to assess the need for resources for inpatient care as well as for outpatient care. Assessing the reasons for inadequacies might lead also to the examination of organizational questions. Finally, a common tool allows comparisons between countries concerning the frequency of inappropriate admissions and days of stay and their reasons in relation to the different organizations of health care across Europe.