This volume collects the papers presented at ICDT 2003, the 9th International ConferenceonDatabaseTheory, heldfrom8to10January2003intheRettorato dell'Universit a di Siena, Siena, Italy. ICDT (http: //alpha.luc.ac.be/ lucp1080/icdt/) has now a long t- dition of international conferences, providing a biennial scienti?c forum for the communicationofhigh-qualityresearchresultsontheoreticalaspectsofallforms of database systems and database technology. ICDT is traditionally held in - storic European locations: Rome in 1986, Bruges in 1988, Paris in 1990, Berlin in 1992, Prague in 1995, Delphi in...