The European Commission supports the development of a European Virtual Coastal and Marine Data Warehouse called CoastBase that aims to improve data and information search and exchange. This paper discusses a study of user requirements and defines relevant and obtainable data and information within the CoastBase project. Thematic as well as technical considerations were taken into account in the study. CoastBase aims to satisfy a variety of users. Potential users interested in data and information on various aggregation levels are represented within the CoastBase consortium. They described their requirements, initially unguided but later guided by a few broad questions about their present work and about the potential role of CoastBase in facilitating it. The results were grouped and summarised. Institutional user groups were formed based on the focus of the organisations on European, regional, national, and local issues, respectively. Individual users were grouped according to their function as Decision & policy maker, Policy advisor & project manager, Researcher, or Database administrator & programmer, respectively. The study has shown that the individual user groups in particular differed in their preferences for aggregation level of the data and information to be extracted from CoastBase. Implications, in the form of envisioned dissimilar use of the CoastBase system are discussed. Both institutional and individual users had different preferences for topic and geographical area to be covered in the CoastBase prototype. However, the study provided a useful synthesis for discussion in plenary, and led to identification of two thematic scenarios for development of the prototype, which are Eutrophication in the North Sea, and Planning in the North Sea and Mediterranean coastal zone. In addition, a comprehensive list of requirements and recommendations for the technical development and evaluation of the system had been compiled from the user requirements. This list, which was approved by the consortium, is included in the paper. © 2003 Elsevier Science Ltd. All rights reserved.