This book constitutes the strictly refereed post-workshop proceedings of the 12th International Workshop on Computer Science Logic, CSL '98, held as the Annual Conference of the European Association on Computer Science Logic in Brno, Czech Republic in August 1998.The 25 revised full papers presented were carefully reviewed and selected during two rounds of reviewing and revision. Also included are three reviewed invited papers. The papers span the whole scope of computer science logic and mathematical foundations and represent the state of the art in the area.

Invited Papers.- Trakhtenbrot Theorem and Fuzzy Logic.- Descriptive Complexity, Lower Bounds and Linear Time.- Testing of Finite State Systems.- Contributed Papers.- On the Power of Quantifiers in First-Order Algebraic Specification.- On the Effective Semantics of Nondeterministic, Nonmonotonic, Temporal Logic Databases.- Revision Programming = Logic Programming + Integrity Constraints.- Quantifiers and the System KE: Some Surprising Results.- Choice Construct and Lindström Logics.- Monadic NP and Graph Minors.- Invariant Definability and P/poly.- Computational Complexity of Ehrenfeucht-Fraïssé Games on Finite Structures.- An Upper Bound for Minimal Resolution Refutations.- On an Optimal Deterministic Algorithm for SAT.- Characteristic Properties of Majorant-Computability Over the Reals.- Theorems of Péter and Parsons in Computer Programming.- Kripke, Belnap, Urquhart and Relevant Decidability & Complexity.- Existence and Uniqueness of Normal Forms in Pure Type Systems with ??-conversion.- Normalization of Typable Terms by Superdevelopments.- Subtyping Functional+Nonempty Record Types.- Monotone Fixed-Point Types and Strong Normalization.- Morphisms and Partitions of V-sets.- Computational Adequacy in an Elementary Topos.- Logical Relations and Inductive/Coinductive Types.- On the Complexity of H-Subsumption.- Complexity Classes and Rewrite Systems with Polynomial Interpretation.- RPO Constraint Solving Is in NP.- Quantifier Elimination in Fuzzy Logic.- Many-Valued First-Order Logics with Probabilistic Semantics.