After two successful MATES conferences in Erfurt 2003 and 2004, the 3rd G- man conference on Multi-agent System Technologies (MATES 2005) took place in Koblenz, Germany, in September 2005, and was co-located with the 28th German Conference on Arti?cial Intelligence (KI 2005). Building onotheragent-relatedeventsin Germanyinthepast, andorganized by the GI German Special Interest Group on Distributed Arti?cial Intelligence, the MATES conference series aims at promoting the theory and applications of agentsandmultiagentsystems.Incorporatingthe9th International Workshopon Cooperative Information Agents (CIA 2005), the topics of interest for MATES 2005 also covered the ?elds of intelligent information agents and systems for the Internet and the (Semantic) Web. As in recent years, MATES 2005 provided a distinguished, lively and int- disciplinary forum for researchers, users, and developers of agent technology, to present and discuss the latest advances of research and development in the area of autonomous agents and multiagent systems. Accordingly, the topics of MATES 2005 covered the whole range from the theory to applications of age- and multiagent technology. The technical program included a total of 24 sci- ti?c talks, and demonstrations of selected running agent systems, and both the MATES 2005 Best Paper and the CIA 2005 System Innovation awards.