We present the Virtual Database - an architecture aiming at the integration of heterogeneous distributed spatial data repositories for comprehensive data visualization, exploration and analysis. Different databases are brought together in order to build an integrated data federation. Data access is based on uniformly defined, standardized and open interfaces conforming to the OpenGIS specifications for Web Feature Services (WFS) and the Geography Markup Language (GML), and selected interoperable technologies proposed and ratified by the W3C Consortium. Visualization, exploration and analysis functionality are provided by the integrated coupling of map server software and extended spatial data handling methods. First experiences with the Virtual Database show its high flexibility and scalability resulting from the interface-based approach and a caching mechanism based on data replication.