This book constitutes the refereed proceedings of the Third International XML Database Symposium, XSym 2005, held in Trondheim, Norway in August 2005 in conjunction with VLDB 2005 and in coordination with the Database and Programming Languages Symposium, DBPL 2005. The 15 revised full papers were carefully reviewed and are preluded by a joint XSym-DBPL keynote talk. The papers are organized in topical sections on indexing support for the evaluation of XPath and XQuery; benchmarks and algorithms for XQuery and XPath evaluation; algorithms for constraint satisfaction checking, information extraction, and subtree matching; and applications of XML in information systems.