Book excerpt: This book constitutes the refereed proceedings of the 9th International Conference on Database Theory, ICDT 2002, held in Siena, Italy in January 2002. The 26 revised full papers presented together with 3 invited articles were carefully reviewed and selected from 92 submissions. The papers are organized in topical sections on reasoning about XML schemas and queries, aggregate queries, query evaluation, query rewriting and reformulation, semistructured versus structured data, query containment, consistency and incompleteness, and data structures.