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."