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 Data Mining Systems, Parallelism and Query Planning, Data Warehousing and Decision Support Systems, Temporal, Spatial and High Dimensional Databases, Data Mining Algorithms, Information Retrieval, Query Processing and Optimization.