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Abstract.Knowledge reuse by means of ontologies faces three important problems at present: (1) there are no standardized identifying features that characterize ontologies from the user point of view; (2) there are no web sites using the same logical organization, presenting relevant information about ontologies; and (3) the search for appropriate ontologies is hard, time-consuming and usually fruitless. To solve the above problems, we present: (1) a living set of features that allow us to characterize ontologies from the user point of view and have the same logical organization; (2) a living domain ontology about ontologies (called Reference Ontology) that gathers, describes and has links to existing ontologies; and (3) (ONTO) 2 Agent, the ontology-based WWW broker about ontologies that uses Reference Ontology as a source of its knowledge and retrieves descriptions of ontologies that satisfy a given set of constraints.

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Abstract. Knowledge reuse by means of outologies now faces three important problems: (1) there are no standardized identifying features that characterize ontologies from the user point of view; (2) there are no web sites using the same logical organization, presenting relevant information about ontologies; and (3) the search for appropriate ontologies is hard, time-consuming and usually fruitless. To solve the above problems, we present: (1) a living set of features that allow us to characterize ontologies from the user point of view and have the same logical organization; (2) a living domain ontology about ontologies (called Reference Ontology) that gathers, describes and has links to existing ontologies; and (3) (ONTO)2Agent, the ontology-based www broker about ontologies that uses the Reference Ontology as a source of its knowledge and retrieves descriptions of ontologies that satisfy a given set of constraints. (ONTO)~Agent is available at http://delicias.dia.fi.upm.es/REFERENCE ONTOLOGY/ 1 INTRODUCTION AND MOTIVATION Nowadays, it is easy to get information from organizations that have ontologies using the WWW. There are even specific points that gather information about ontologies and have links to other web pages containing more explicit information about such ontologies (see The Ontology Page 4, also known as TOP) and there are also ontology servers like The Ontology Server s [8, 9], Cycorp's Upper CYC Ontology Server 6 [29] or Ontosaurus 7 [36] that collect a huge number of very well-known ontologies.When developers search for candidate ontologies for their application, they face a complex multi-criteria choice problem. Apart from the dispersion of ontologies over several servers; (a) ontology content formalization differs depending on the server at which it is stored; (b) ontologies on the same server are usually described with different detail levels; and (c) there is no common format for presenting relevant information about the ontologies so that users can decide which ontology best stilts their purpose.Choosing an ontology that does not match tile system needs properly or whose usage is expensive (people, hardware and software resources, time) may force future users to stop reusing the ontology already built and oblige them to formalize the same knowledge