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 1997, Jerusalem in 1999, and London in 2001. ICDThasmergedwiththeSymposiumonMathematicalFundamentalsofDa- base Systems (MFDBS), initiated in Dresden in 1987, and continued in Visegrad in 1989 and Rostock in 1991. This volume contains 26 papers selected from 92 submissions, plus three invited papers by Hector Garcia-Molina, Yannis Ioannidis, and Limsoon Wong. We wish to thank all the authors who submitted papers, the members of the program committee for their e?orts in reviewing and selecting the papers, the external referees, the organizing committee, and the sponsors for their support. Diego Calvanese Maurizio Lenzerini Rajeev Motwani Organization ICDT 2003 was organized by the Dipartimento di Informatica e Sistemistica, Universit' a di Roma "La Sapienza".