Invited Papers -- Perspectives on Transactional Memory -- The Effect of Tossing Coins in Omega-Automata -- Bigraphical Categories -- BlenX Static and Dynamic Semantics -- Contributed Papers -- Flow Policy Awareness for Distributed Mobile Code -- Volume and Entropy of Regular Timed Languages: Discretization Approach -- A Logical Interpretation of the ?-Calculus into the ?-Calculus, Preserving Spine Reduction and Types -- Encoding Asynchronous Interactions Using Open Petri Nets -- Distributed Asynchronous Automata -- Algebra for Infinite Forests with an Application to the Temporal Logic EF -- Deriving Syntax and Axioms for Quantitative Regular Behaviours -- Weighted Bisimulation in Linear Algebraic Form -- A Logic-Based Framework for Reasoning about Composite Data Structures -- Measuring Permissivity in Finite Games -- Contracts for Mobile Processes -- Power of Randomization in Automata on Infinite Strings -- Probabilistic Weighted Automata -- Partially-Commutative Context-Free Processes -- Testing Finitary Probabilistic Processes -- A Demonic Approach to Information in Probabilistic Systems -- HYPE: A Process Algebra for Compositional Flows and Emergent Behaviour -- Variable Binding, Symmetric Monoidal Closed Theories, and Bigraphs -- Partial Order Reduction for Probabilistic Systems: A Revision for Distributed Schedulers -- Model-Checking Games for Fixpoint Logics with Partial Order Models -- Reachability in Succinct and Parametric One-Counter Automata -- Winning Regions of Pushdown Parity Games: A Saturation Method -- Concurrent Kleene Algebra -- Concavely-Priced Probabilistic Timed Automata -- Compositional Control Synthesis for Partially Observable Systems -- Howe's Method for Calculi with Passivation -- On the Relationship between ?-Calculus and Finite Place/Transition Petri Nets -- ModelingRadio Networks -- Time-Bounded Verification -- Secure Enforcement for Global Process Specifications -- On Convergence of Concurrent Systems under Regular Interactions -- Computing Stuttering Simulations -- Weak Time Petri Nets Strike Back! -- A General Testability Theory -- Counterexamples in Probabilistic LTL Model Checking for Markov Chains -- Query-Based Model Checking of Ad Hoc Network Protocols -- Strict Divergence for Probabilistic Timed Automata.

This book constitutes the refereed proceedings of the 20th International Conference on Concurrency Theory, CONCUR 2009, held in Bologna, Italy, September 1-4, 2009. The 37 revised full papers presented together with four invited papers were carefully reviewed and selected from 129 submissions. The topics include model checking, process calculi, minimization and equivalence checking, types, semantics, probability, bisimulation and simulation, real time, and formal languages.