A detailed requirements analysis is best practice in the development of traditional software. Conversely, the importance of requirements engineering for Web systems is still underestimated. Only few Web methodologies provide an approach for the elicitation of requirements and techniques for their specification. This paper focuses on specification through requirements models of Web systems. We present a metamodel, which contains the key concepts needed for the requirements specification of Web systems. The benefit of such a metamodel is twofold: (1) The key concepts are used for the definition of a common modeling language: a UML profile for Web requirements. (2) The elements of the metamodel are mapped to the modeling constructs of the different Web methodologies. In this way the prerequisite for model-to-model transformations is given, which allows to build different views of the requirements of a Web system using different Web methodologies. * This research has been partially supported by the project Nido of the Ministerio de Ciencia y Tecnología (TIC 2003-369) and the Junta de Andalucía, Spain, the project MAEWA “Model Driven Development of Web Applications” (WI841/7-1) of the Deutsche Forschungsgemeinschaft (DFG), Germany, and the EC 6th Framework project SENSORIA “Software Engineering for Service-Oriented Overlay Computers” (IST 016004).