This volume contains the presentations of the Sixth Symposium on Theoretical Aspects of Computer Science (STACS 89) held at the University of Paderborn, February 16-18, 1989. In addition to papers presented in the regular program the volume contains abstracts of software systems demonstrations which were included in this conference series in order to show applications of research results in theoretical computer science. The papers are grouped into the following thematic sections: computational geometry, automata theory and formal languages, semantics of programming languages, parallel algorithms, graph algorithms, complexity, structures, fault tolerance, completeness, distributed computing and concurrency.