||Data warehouses have become the enabling technology for strategic business applications. The continuous growth of enterprises as well as the rapid development of their information technology, however, led to islands of data warehouses, which in many enterprises are operated separately and lack adequate integration. From a large data warehouse project we are conducting with the Upper-Austrian Health Insurance Organization we recognize the demand for making the existing data warehouse solutions interoperable. We thereby have the goal to overcome distribution and heterogeneity problems while preserving the autonomy of the particular data warehouses and their applications.In order to make these islands of data warehouses interoperable, we propose a federated environment being an extensible integration platform where individual data warehouses may participate while continuing their current applications. In this paper we describe the main components needed for a federated data warehouse and explore the challenges related to developing and operating such a system. In particular, we address issues concerning schema integration, query processing, data exchange, and mediation between the local and the federated data warehouse layers.