||Providing integrated access to multiple, heterogeneous, distributed, and autonomous data warehouses has become one of the leading issues in data warehouse research and industry. From a large data warehouse project we are conducting with the Upper-Austrian Health Insurance Organization we recognize the demand for making "islands" of data warehouse solutions interoperable to better support high-level strategic business applications. This paper presents a framework for providing interoperability of distributed, heterogeneous, and autonomous data warehouses based on a federated approach. We describe the main issues related to implementing such a federated data warehouse including the topics schema integration, query processing, data exchange, and mediation between the local and the federated data warehouse layers. We then focus on schema integration using XML, which offers significant benefits for interoperability, since it is a standardized, vendor and platform independent format for data and - in our case - metadata exchange.