M.A. BRAMER University of Portsmouth, UK This volume comprises the refereed technical papers presented at ES200 l, the Twenty-fIrst SGES International Conference on Knowledge Based Systems and Applied ArtifIcial Intelligence, held in Cambridge in December 200 l, together with an invited keynote paper by Professor Derek Sleeman. The conference was organised by SGES, the British Computer Society Specialist Group on Knowledge Based Systems and Applied ArtifIcial Intelligence. The papers in this volume present new and innovative developments in the fIeld, divided into sections on Machine Learning, Constraint Satisfaction, Agents, Knowledge Representation, Knowledge Engineering, and Intelligent Systems. The refereed papers begin with a paper entitled 'Detecting Mismatches Among Experts' Ontologies Acquired Through Knowledge Elicitation', which describes a systematic approach to the analysis of discrepancies within and among experts' ontologies. This paper was judged to be the best refereed technical paper submitted to the conference. The remaining papers are devoted to topics in important areas such as agents, knowledge engineering, knowledge representation, planning and constraint satisfaction, with machine learning again the largest topic covered in terms of the number of papers accepted for publication. This is the eighteenth volume in the Research and Development series. The Application Stream papers are published as a companion volume under the title Applications and Innovations in Intelligent Systems IX.