This special issue presents a selection of papers in Knowledge Representation (KR) in Artificial Intelligence (AI), intended to illustrate the depth and breadth of current research in the area. It comes just over 25 years since a similar special issue of the Journal of Philosophical Logic appeared on the topic Philosophical Logic and Artificial Intelligence [15] . This latter special issue covered work addressing the use of logic, in one form or another, for representing and reasoning with

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... edge. The papers of the 1988 special issue give a good indication of major themes of research at the time: reasoning about belief and knowledge [5] , tractable logical reasoning [10], nonmonotonicity [11] , and reasoning about action [13] . These topics can be considered to be centrally in KR; the current issue then also serves to indicate how the field has broadened and evolved over the last 25 years. Traditionally the area of Knowledge Representation has been seen as "the field of study concerned with using formal symbols to represent a collection of propositions believed by some putative agent" [3] . (A further, key, aspect of KR is reasoning since, after all, KR is an area of Artificial Intelligence which in turn is concerned with the implementation of intelligent artifacts on a computer.) However, KR has grown to the extent that this traditional characterisation is overly limiting. A more general definition, which better fits the current state of the art, as reflected by the coverage of J. Delgrande ( )