TACS'91 is the first International Conference on Theoretical Aspects of Computer Science held at Tohoku University, Japan, in September 1991. This volume contains 37 papers and an abstract for the talks presented at the conference. TACS'91 focused on theoretical foundations of programming, and theoretical aspects of the design, analysis and implementation of programming languages and systems. The following range of topics is covered: logic, proof, specification and semantics of programs and languages; theories and models of concurrent, parallel and distributed computation; constructive logic, category theory, and type theory in computer science; theory-based systems for specifying, synthesizing, transforming, testing, and verifying software.