This volume contains the proceedings of STACS 91, a symposium on the theoretical aspects of computer science, held in Hamburg, Germany in February 1991. STACS is held each year, alternately in Germany and France, and is organized jointly by the Special Interest Group for Theoretical Computer Science of the Gesellschaft fuer Informatik (GI) and the Special Interest Group for Applied Mathematics of the Association Francaise des Sciences et Techniques de l'Information et de Systemes (AFCET). The topics covered in this volume include abstract data types, algorithms and data structures, automata and formal languages, complexity of concrete algorithms, computational geometry, cryptography, computer systems theory, logic and semantics, mathematics of computation, program specification, theory of parallel and distributed computation, structural complexity, theory of robotics, VLSI structures, and the theory of databases.