During the development of a knowledge-based audio- visual information system the authors of this article defined a conceptual system architecture based on MPEG-7 as the general description scheme for the media assets in the middleware. This concept was not only used to achieve a high abstraction and independence of the underlying media asset management system, it was also and primarily used as the basis of a semantic indexing process. Based on lightweight ontologies the descriptions of the media assets were associated with semantic concepts. Semantically annotated MPEG-7 assets were then propagated to the presentation layer, thus allowing the implementation of a variety of publication scenarios, including crossmedia scenarios for the creation of concise video summaries.