This book constitutes the refereed proceedings of the 9th Internationa l Conference on Artificial Intelligence: Methodology, Systems, and App lications, AIMSA 2000, held in Varna, Bulgaria in September 2000. The 34 revised full papers presented were carefully reviewed and selected from 60 submissions. The papers are organized in topical sections on k nowledge construction, reasoning under certainty, reasoning under unce rtainty, actors and agents, Web mining, natural language processing, c omplexity and optimization, fuzzy and neural systems, and algorithmic learning.