Today, the Web is increasingly used as a platform for distributed services, which transcend organizational boundaries to form federated applications. Consequently, there is a growing interest in the architectural aspect of Web-based systems, i.e. the composition of the overall solution into individual Web applications and Web services from different parties. The design and evolution of federated systems calls for models that give an overview of the structural as well as trust-specific composition and reflect the technical details of the various accesses. We introduce the WebComposition Architecture Model (WAM) as an overall modeling approach tailored to aspects of highly distributed systems with federation as an integral factor. Categories and Subject Descriptors D.2.11 [Software Engineering]: Software Architectures ? Languages; H.3.4 [Information storage and retrieval]: Systems and Software ? Distributed systems General Terms Management, Design, Security Keywords Security, Web Servic...