Invited Presentations -- Functorial Boxes in String Diagrams -- Some Results on a Game-Semantic Approach to Verifying Finitely-Presentable Infinite Structures (Extended Abstract) -- Automata and Logics for Words and Trees over an Infinite Alphabet -- Nonmonotonic Logics and Their Algebraic Foundations -- Contributions -- Semi-continuous Sized Types and Termination -- Visibly Pushdown Automata: From Language Equivalence to Simulation and Bisimulation -- A Finite Semantics of Simply-Typed Lambda Terms for Infinite Runs of Automata -- The Power of Linear Functions -- Logical Omniscience Via Proof Complexity -- Verification of Ptime Reducibility for System F Terms Via Dual Light Affine Logic -- MSO Queries on Tree Decomposable Structures Are Computable with Linear Delay -- Abstracting Allocation -- Collapsibility in Infinite-Domain Quantified Constraint Satisfaction -- Towards an Implicit Characterization of NC k -- On Rational Trees -- Reasoning About States of Probabilistic Sequential Programs -- Concurrent Games with Tail Objectives -- Nash Equilibrium for Upward-Closed Objectives -- Algorithms for Omega-Regular Games with Imperfect Information -- Relating Two Standard Notions of Secrecy -- Jump from Parallel to Sequential Proofs: Multiplicatives -- First-Order Queries over One Unary Function -- Infinite State Model-Checking of Propositional Dynamic Logics -- Weak Bisimulation Approximants -- Complete Problems for Higher Order Logics -- Solving Games Without Determinization -- Game Quantification on Automatic Structures and Hierarchical Model Checking Games -- An Algebraic Point of View on the Crane Beach Property -- A Sequent Calculus for Type Theory -- Universality Results for Models in Locally Boolean Domains -- Universal Structures and the Logic of Forbidden Patterns -- On the Expressive Power of Graph Logic -- Hoare Logic in the Abstract -- Normalization of IZF with Replacement -- Acyclicity and Coherence in Multiplicative Exponential Linear Logic -- Church Synthesis Problem with Parameters -- Decidable Theories of the Ordering of Natural Numbers with Unary Predicates -- Separation Logic for Higher-Order Store -- Satisfiability and Finite Model Property for the Alternating-Time ?-Calculus -- Space-Efficient Computation by Interaction -- The Ackermann Award 2006.

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.