The annual conference of the European Association for Computer Science Logic (EACSL), CSL 2009, was held in Coimbra (Portugal), September 711, 2009. The conference series started as a programme of International Workshops on Computer Science Logic, and then at its sixth meeting became the Annual C- ference of the EACSL. This conference was the 23rd meeting and 18th EACSL conference; it was organized at the Department of Mathematics, Faculty of S- ence and Technology, University of Coimbra. In response to the call for papers, a total of 122 abstracts were submitted to CSL 2009of which 89 werefollowedby a full paper. The ProgrammeCommittee selected 34 papers for presentation at the conference and publication in these proceedings. The Ackermann Award is the EACSL Outstanding Dissertation Award for Logic in Computer Science. The awardrecipient for 2009 was Jakob Nordstr¨ om. Citation of the award, abstract of the thesis, and a biographical sketch of the recipient may be found at the end of the proceedings. The award was sponsored for the years 20072009 by Logitech S.A.

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Invited Talks.- Algebra for Tree Languages.- Forcing and Type Theory.- Functional Interpretations of Intuitionistic Linear Logic.- Fixed-Point Definability and Polynomial Time.- Special Invited Talk to Commemorate the Centenary of Stephen Cole Kleene.- Kleene's Amazing Second Recursion Theorem.- Contributed Papers.- Typed Applicative Structures and Normalization by Evaluation for System F ? .- Jumping Boxes.- Tree-Width for First Order Formulae.- Algorithmic Analysis of Array-Accessing Programs.- Decidable Relationships between Consistency Notions for Constraint Satisfaction Problems.- Cardinality Quantifiers in MLO over Trees.- From Coinductive Proofs to Exact Real Arithmetic.- On the Relation between Sized-Types Based Termination and Semantic Labelling.- Expanding the Realm of Systematic Proof Theory.- EXPTIME Tableaux for the Coalgebraic ?-Calculus.- On the Word Problem for -Categories, and the Properties of Two-Way Communication.- Intersection, Universally Quantified, and Reference Types.- Linear Game Automata: Decidable Hierarchy Problems for Stripped-Down Alternating Tree Automata.- Enriching an Effect Calculus with Linear Types.- Degrees of Undecidability in Term Rewriting.- Upper Bounds on Stream I/O Using Semantic Interpretations.- Craig Interpolation for Linear Temporal Languages.- On Model Checking Boolean BI.- Efficient Type-Checking for Amortised Heap-Space Analysis.- Deciding the Inductive Validity of ????* Queries.- On the Parameterised Intractability of Monadic Second-Order Logic.- Automatic Structures of Bounded Degree Revisited.- Nondeterminism and Observable Sequentiality.- A Decidable Spatial Logic with Cone-Shaped Cardinal Directions.- Focalisation and Classical Realisability.- Decidable Extensions of Church's Problem.- NestedHoare Triples and Frame Rules for Higher-Order Store.- A Complete Characterization of Observational Equivalence in Polymorphic ?-Calculus with General References.- Non-Commutative First-Order Sequent Calculus.- Model Checking FO(R) over One-Counter Processes and beyond.- Confluence of Pure Differential Nets with Promotion.- Decision Problems for Nash Equilibria in Stochastic Games.- On the Complexity of Branching-Time Logics.- Nominal Domain Theory for Concurrency.- The Ackermann Award 2009.