The papers in this volume are the refereed technical papers presented at AI2003, the Twenty-third SGAI International Conference on theory, practical and application of Artificial Intelligence, held in Cambridge in December 2003. The papers in this volume present new and innovative developments in the field, divided into sections on Machine Learning, Knowledge Representation and Reasoning, Knowledge Acquisition, Constraint Satisfaction and Scheduling, and Natural Language Processing. This is the twentieth volume in the Research and Development series. The series is essential reading for those who wish to keep up to date with developments in this important field. The Application Stream papers are published as a companion volume under the title Applications and Innovations in Intelligent Systems XI.