This book constitutes the refereed proceedings of the 27th Annual German Conference on Artificial Intelligence, KI 2004, held in Ulm, Germany, in September 2004.The 29 revised full papers presented together with 5 invited contributions were carefully reviewed and selected from 103 submissions. The papers are organized in topical sections on natural language processing, knowledge representation and ontologies, planning and search, neural networks and machine learning, reasoning, and robotics and machine perception.