This book constitutes the refereed proceedings of the 28th Annual German Conference on Artificial Intelligence, KI 2005, held in Koblenz, Germany, in September 2005 - co-located with the 3rd German Conference on Multiagent System Technologies (MATES 2005). The 29 revised full papers presented together with 3 invited contributions were carefully reviewed and selected from 113 submissions. The papers are organized in topical sections on knowledge representation and reasoning, machine learning, diagnosis, neural networks, planning, robotics, and cognitive modeling, philosopy, natural language.