This volume contains the proceedings of ICALP '91, the 18th annual summer conference sponsored by the European Association for Theoretical Computer Science (EATCS). ICALP stands for International Colloquium on Automata, Languages, and Programming, and this conference series covers all important areas of theoretical computer science, such as: computability, automata, formal languages, data types and structures, theory of databases and knowledge bases, semantics of programming languages, program specification, transformation and verification, foundations of logic and functional programming, theory of logical design and layout, parallel and distributed computation, theory of concurrency, symbolic and algebraic computation, term rewriting systems, computational geometry, cryptography, and theory of robotics.