This book constitutes the refereed proceedings of the 9th International Conference on Extending Database Technology, EDBT 2004, held in Heraklion, Crete, Greece, in March 2004. The 42 revised full papers presented together with 2 industrial application papers, 15 software demos, and 3 invited contributions were carefully reviewed and selected from 294 submissions. The papers are organized in topical sections on distributed, mobile and peer-to-peer database systems; data mining and knowledge discovery; trustworthy database systems; innovative query processing techniques for XML data; data and information on the web; query processing techniques for spatial databases; foundations of query processing; advanced query processing and optimization; query processing techniques for data and schemas; multimedia and quality-aware systems; indexing techniques; and imprecise sequence pattern queries.