This book constitutes the refereed proceedings of the 13th Annual International Conference on Computer Science Logic, CSL '99, held under the sponsorship of EACSL in Madrid, Spain in September 1999.The 34 revised full papers presented together with five invited contributions were carefully reviewed and sleected from a total of 91 submissions. The papers are organized in sections on descriptive complexity; verification; temporal logic; lambda calculus and linear logic; logic programming, modal logic, and description logic; logic and complexity; lambda calculus and type theory; linear logic, mu calculus, and concurrency; and specification and data refinement.