The 9th Italian Conference on Theoretical Computer Science (ICTCS 2005)was held at the Certosa di Pontignano, Siena, Italy, on October 12-14 2005. The Certosa di Pontignano is the conference center of the University of Siena; it is located 8 km away from the town and it is in the Chianti region. The Certosa is a place full of history (founded in the 15th century, it was set on ?re a century later and reconstructed) and of valuable artworks, like frescoes of the Scuola Senese. Previous conferences took place in Pisa (1972), Mantova (1974 and 1989), L'Aquila(1992),Ravello(1995),Prato(1998),Turin(2001)andBertinoro(2003). Theconferenceaimsatbringingtogethercomputerscientists,especiallyyoung researchers, to foster cooperation, exchange of ideas and results. Great e?orts have been made to attract researchers from all over the world. The main t- ics of the conference cover all the ?elds of theoretical computer science and include analysis and design of algorithms, computability, computational c- plexity, cryptography, formal languages and automata, foundations of progr- ming languages and program analysis, foundations of arti?cial intelligence and knowledge representation, foundations of web programming, natural computing paradigms (quantum computing, bioinformatics), parallel and distributed c- putation, program speci? cation and veri?cation, term rewriting, theory of c- currency, theory of data bases, theory of logical design and layout, type theory, security, and symbolic and algebraic computation.