This volume contains the refereed technical papers presented at ES99, the Nineteenth SGES International Conference on Knowledge-Based Systems and Applied Artificial Intelligence, held in Cambridge in December 1999. The papers in this volume present new and innovative developments in the field, divided into sections on knowledge engineering, knowledge discovery, case-based reasoning, learning and knowledge representation and refinement. This is the sixteenth 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 VII.