Abstract. The fields of philosophy and informatics entertain two somewhat different conceptions of ontology. Philosophical ontology is a branch of metaphysics dating back at least to the time of Plato and Aristotle. Ontology in informatics has its origins in the artificial intelligence research of the eighties and nineties. This means that the fields of philosophy and informatics entertain two somewhat different conceptions of ontology and the present paper discusses the relationship between these two conceptions. Differences and similarities are pointed out and variations in methodological approaches are also discussed. Efforts to combine the ontological methodologies and resources of the two fields are surveyed, and actual and potential benefits and drawbacks of such collaborations are examined. Different Concepts The fields of philosophy and informatics entertain two somewhat different conceptions of ontology, with correspondingly different notions of what ontology is for. First, we...