This book constitutes the refereed proceedings of the 13th International Conference on Artificial Intelligence: Methodology, Systems, and Applications, AIMSA 2008, held in Varna, Bulgaria in September 2008. The 30 revised full papers presented together with the 10 posters were carefully reviewed and selected from 109 submissions. The papers are organized in topical sections on agents; natural language processing and text analysis; machine learning and information retrieval; knowledge representation and reasoning; constraints, heuristics and search; applications; posters.