This book constitutes the refereed proceedings of the 20th International Conference on Database and Expert Systems Applications, DEXA 2009, held in Linz, Austria, in August/September 2009.The 35 revised full papers and 35 short papers presented were carefully reviewed and selected from 202 submissions. The papers are organized in topical sections on XML and databases; Web, semantics and ontologies; temporal, spatial, and high dimensional databases; database and information system architecture, performance and security; query processing and optimisation; data and information integration and quality; data and information streams; data mining algorithms; data and information modelling; information retrieval and database systems; and database and information system architecture and performance.