This book constitutes the refereed proceedings of the 23rd Annual German Conference on Artificial Intelligence, KI-99, held in Bonn, Germany, in September 1999. The volume opens with six full invited survey papers by internationally leading authorities on the subject. The 15 revised full papers and the six short papers presented were carefully reviewed and selected from 32 submissions received. Besides the established traditional topics addressed, special emphasis is placed on pattern recognition and robotics from an AI point of view.