This book constitutes the refereed proceedings of the 8th International Conference on Theoretical Computer Science, ICTCS 2003, held in Bertinoro, Italy in October 2003. The 27 revised full papers presented together with an invited paper and abstracts of 2 invited talks were carefully reviewed and selected from 65 submissions. The papers are organized in topical sections on program design-models and analysis, algorithms and complexity, semantics and formal languages, and security and cryptography.