Invited Talks -- Facets of Synthesis: Revisiting Church's Problem -- Temporal Reasoning about Program Executions -- Semantics -- Least and Greatest Fixpoints in Game Semantics -- Full Abstraction for Reduced ML -- Logics and Bisimulation Games for Concurrency, Causality and Conflict -- Logics and Automata -- Separating Graph Logic from MSO -- On the Completeness of Dynamic Logic -- Dependency Tree Automata -- On Global Model Checking Trees Generated by Higher-Order Recursion Schemes -- Algebras -- A Kleene Theorem for Polynomial Coalgebras -- Coalgebraic Hybrid Logic -- A Description of Iterative Reflections of Monads (Extended Abstract) -- Automata Theory -- Tighter Bounds for the Determinisation of Büchi Automata -- Lower Bounds on Witnesses for Nonemptiness of Universal Co-Büchi Automata -- Interrupt Timed Automata -- Parameter Reduction in Grammar-Compressed Trees -- Processes and Models -- The Calculus of Handshake Configurations -- On the Expressive Power of Restriction and Priorities in CCS with Replication -- Normal Bisimulations in Calculi with Passivation -- Reactive Systems, Barbed Semantics, and the Mobile Ambients -- Security -- On the Foundations of Quantitative Information Flow -- Cryptographic Protocol Composition via the Authentication Tests -- Bisimulation for Demonic Schedulers -- Probabilistic and Quantitative Models -- On Omega-Languages Defined by Mean-Payoff Conditions -- Minimal Cost Reachability/Coverability in Priced Timed Petri Nets -- Delayed Nondeterminism in Continuous-Time Markov Decision Processes -- Concurrency, ?-Algebras, and Probabilistic Fairness -- Synthesis -- Synthesis from Component Libraries -- Realizability of Concurrent Recursive Programs -- Program Analysis and Semantics -- Beyond Shapes: Lists with Ordered Data -- Interprocedural Dataflow Analysis over WeightDomains with Infinite Descending Chains -- Realizability Semantics of Parametric Polymorphism, General References, and Recursive Types.

This book constitutes the refereed proceedings of the 12th International Conference on Foundations of Software Science and Computational Structures, FOSSACS 2009, held in York, UK, in March 2009, as part of ETAPS 2009, the European Joint Conferences on Theory and Practice of Software. The 30 revised full papers presented together with two invited talks were carefully reviewed and selected from 102 full paper submissions. The topics addressed are semantics, logics and automata, algebras, automata theory, processes and models, security, probabilistic and quantitative models, synthesis, and program analysis and semantics.