This volume gives the proceedings of the ninth Symposium onTheoretical Aspects of Computer Science (STACS). Thisannual symposium is held alternately in France and Germanyand is organized jointly by the Special Interest Group forFundamental Computer Science of the Association Francaisedes Sciences et Technologies de l'Information et desSyst mes (AFCET) and the Special Interest Group forTheoretical Computer Science of the Gesellschaft frInformatik (GI). The volume includes three invited lectures and sections onparallel algorithms, logic and semantics, computationalgeometry, automata and languages, structural complexity,computational geometry and learning theory, complexity andcommunication, distributed systems, complexity, algorithms,cryptography, VLSI, words and rewriting, and systems.