The papers in this volume are the refereed technical papers presented at ES98, the Eighteenth Annual International Conference of the British Computer Society Specialist Group on Expert Systems, held in Cambridge in December 1998. The papers in this volume present new and innovative developments within the field of expert systems and artificial intelligence. For example, papers on expert systems construction, validation and verification of knowledge bases, reasoning about device functionality, spatial reasoning, machine learning, case-based reasoning and data mining area are included. This is the fifteenth volume in the Research and Development in Expert Systems series. The series is essential reading for those who wish to keep up to date with developments and opportunities in this very important field. The Applications Stream papers are published as a companion volume under the title Applications and Innovations in Expert Systems VI.