In order to ensure interoperability between different urban data bases, a solution based on an ontology is interesting because it allows dealing with semantics. An ontology can be defined as a semantic graph regrouping all concepts which can be found in cities and in urban planning. Classical concepts can range from streets, parcels, rivers, to violence, immigration, transportation, security, etc. The goal of this workshop will be: to define ontologies and explain their use in several domains; • to develop the importance of urban ontologies, especially for providing interoperability between several urban databases, and cooperation between several administrative systems; • to present rapidly the different formalisms and languages for writing ontologies, giving more importance on languages based on XML; • to explain how to develop an ontology, and to reach to some consensus between several stakeholders ; several methodologies will be explained and compared. The end of this workshop will be devoted to the presentation of the Towntology project developed in Lyon (presently around 1000 concepts in French language), and the preparation of a COST European group in order to extend the existing ontologies to other urban aspects so to cover the whole urban fields, and also to get it in different languages. Dr. Jacques Teller of the University of Liege, Belgium is in charge of the constitution of this COST group. See http://lisi.insa-lyon.fr/~townto.