TACS'91 is the first International Conference on TheoreticalAspects of Computer Science held at Tohoku University,Japan, in September 1991. This volume contains 37 papers andan abstract for the talks presented at the conference. TACS'91 focused on theoretical foundations of programming,and theoretical aspects of the design, analysis andimplementation of programming languages and systems. Thefollowing range of topics is covered: logic, proof,specification and semantics of programs and languages;theories and models of concurrent, parallel and distributedcomputation; constructive logic, category theory, and typetheory in computer science; theory-based systems forspecifying, synthesizing, transforming, testing, andverifying software.