One aspect of modern Web technology is that it is not only employed to connect users to sites, but to an increasing degree to bring together applications, as e.g. with the help of Web services. Particularly, these applications can reside under the control of different organizations. In this case, the Web is actually used as a means for establishing federations between the organizations behind their technical systems. This enables creating integrated business processes and thus facilitates seamless cooperation between multiple businesses. Naturally, the construction of such solutions raises specific concerns that go beyond traditional approaches, including questions of access control, identity management and evolution in the context of federation. Emerging federation specifications like WS-Federation or Liberty Alliance are now offering standardized protocols as solutions to some of these problems. Along with the technological base, the process of establishing and operating federations also requires dedicated models to plan and describe the systems on an architectural level. Conventional approaches to modeling software in general and Web applications in particular do not specifically focus on federated architectures. For that purpose, we propose the WebComposition Architecture Model (WAM), a modeling formalism based on our experience on developing federations of Web applications as well as the concepts found in recent federation specifications.