This book constitutes the refereed proceedings of the 20th International Workshop on Computer Science Logic, CSL 2006, held as the 15th Annual Conference of the EACSL in Szeged, Hungary in September 2006.

The 37 revised full papers presented together with 4 invited contributions were carefully reviewed and selected from 132 submissions. All current aspects of logic in computer science are addressed, including automated deduction and interactive theorem proving, constructive mathematics and type theory, equational logic and term rewriting, automata and formal logics, modal and temporal logic, model checking, logical aspects of computational complexity, finite model theory, computational proof theory, logic programming and constraints, lambda calculus and combinatory logic, categorical logic and topological semantics, domain theory, database theory, specification, extraction and transformation of programs, logical foundations of programming paradigms, verification of security protocols, linear logic, higher-order logic, nonmonotonic reasoning, as well as logics and type systems for biology.