This book constitutes the refereed proceedings of the 21 International Conference on Database and Expert Systems Applications, DEXA 2010, held in Bilbao, Spain, August 30 - September 3, 2010. The 45 revised full papers and 36 short papers were carefully reviewed and selected from 197 submissions. The papers are organized in topical sections on Web, Semantics and Ontologies, Information Integration and Process Modeling, Streams and Multimedia Databases, Data Management Algorithms and Performance, Decision Support Systems and Performance, Data Streams, XML Databases, Programming Language and Cooperative Work, Query Processing and Optimization, Data Privacy and Security, Temporal, Spatial, and High Dimensions Databases, Semantic Web and Ontologies.