This book constitutes the thoroughly refereed post-conference proceedings of the 6th Congress of the Italian Association for Artificial Intelligence, AI*IA 99, held in Bologna, Italy, in September 1999. The 33 revised full papers presented were carefully reviewed and selected for inclusion in the book from a total of 64 congress submissions. The papers are organized in topical sections on knowledge representation; automated reasoning; temporal and qualitative reasoning; machine learning, data mining, and theory revision; natural language processing and web interfaces; multi-agent systems; perception and robotics; and planning and scheduling.