This book constitutes the refereed proceedings of the 26th Annual German Conference on Artificial Intelligence, KI 2003, held in Hamburg, Germany in September 2003.The 42 revised full papers presented together with 5 invited papers were carefully reviewed and selected from 90 submissions from 22 countries. The papers are organized in topical sections on logics and ontologies, cognitive modeling, reasoning methods, machine learning, neural networks, reasoning under uncertainty, planning and constraints, spatial modeling, user modeling, and agent technology.