CSL 2009, the 18th Annual Conference (and 23rd International Workshop) of the European Association for Computer Science Logic, was held in Coimbra, Portugal, in September 2009. This special issue contains extended versions of papers presented at the conference.

The six papers of this collection were invited by the guest editors, and cover a wide range of topics represented at the CSL conference, including co-algebraic methods in computer science, finite model theory, intuitionistic and linear logic, model checking, mu-calculus, proof complexity, parametrized complexity, and real number computation. We are grateful to the authors for their excellent submissions. All papers were refereed according to the usual high standards of LMCS.

We would like to express our gratitude to the Programme Committee and the external reviewers for their help in selecting an excellent programme for CSL 2009, and to the authors and participants for their contributions to the success of the conference. Finally, we would like to thank the editors of LMCS for their support to publish this special issue.