Ontologies in Ubiquitous Computing José R. Hilera 1 , Francisco Ruiz 2 1 Department of Computer Science, University of Alcala, Spain. jose.hilera@uah.es 2 Department of Information Technologies and Systems, University of Castilla-La Mancha, Spain. francisco.ruizg@uclm.es Abstract. Ontologies are an explicit specification of a conceptualization, that is understood to be an abstract and simplified version of the world to be repre- sented. In recent years, ontologies have been used in Ubiquitous Computing, especially for the development of context-aware applications. In this paper, we offer a taxonomy for classifying ontologies used in Ubiquitous Computing, in which two main categories are distinguished: Domain ontologies, created to represent and communicate agreed knowledge within some sub-domain of Ubiquitous Computing; and Ontologies as software artifacts, when ontologies play the role of an additional type of artifact in ubiquitous computing applications. The latter category is subdivided according with the moment in that ontologies are used: at development time or at run time. Also, we analyze and classify (based on this taxonomy) some recently published works. Key Words. Ubiquitous Computing, Ontology, Knowledge Domain, Context- Aware Computing. 1. Introduction Although a generally accepted meaning of the term “Ubiquitous Computing” does not exist, it is usual to consider it as a concept that refers to what Weiser has described as “methods of enhancing computer use by making many computers available throughout the physical environment, but making them effectively invisible to the user” [1]. Since the end of the eighties of the last century, the evolution of Ubiquitous Computing has given place to new terms, like “Mobile Computing”, “Wearable Computing”, “Pervasive Computing”, “Context-Aware Computing” or “Tangible Computing”; presents in the lists of topics of the most important congresses and journals about the matter. Nevertheless, not all the researchers in this field agree in the meaning of these terms. For example, for many authors the terms "Pervasive Computing" and "Ubiquitous Computing" are synonymous while for others, as Lyytinen and Yoo [2], represent different concepts because the second implies a higher level of mobility that the first one. This difference of interpretation is mainly due to the fact that it is a very new scientific discipline, and this situation affects also the researchers when searching and