A unified representation for web data and web resources, is absolutely necessary in nowdays large scale Internet data management systems. This representation will allow for the machines to meaningfully process the available information and provide semantically correct answers to imposed queries. Ontologies are expected to play an important role towards this direction of web technology which defines the so called, Semantic Web. The goal of this paper is to provide an overview of the Knowledge Representation (KR) techniques and languages that can be used as standards in the Semantic Web. This work is supported by the bilateral research collaboration programme Platon between Greece and France, 2002–2004 financed by the French and Greek Secretariat of Research and Technology, Ministry of Development.