This book constitutes the refereed proceedings of the 20th International Workshop on Computer Science Logic, CSL 2006. The book presents 37 revised full papers together with 4 invited contributions, addressing all current aspects of logic in computer science. Coverage includes 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, finite model theory, and more.