Dorota Siwecka Knowledge Organization systems used in European national libraries towards interoperability of the semantic Web Abstract The paper presents how European national libraries use knowledge organization systems in projects connected with providing open access to their bibliographic and authority data on the Internet. Over the last ten years there have been two main trends related to the implementation of these projects in order to bring well organized bibliographic and authority data to the world of the Semantic Web. The first trend has been to publish national bibliographic and authority data as open data freely available to everyone on the Internet. The second new approach has been to link open data with other data available on the Web. Performing these kinds of tasks of opening and linking bibliographic and authority data, libraries are involved in creating new tools, among them ontologies, schemas and dictionaries, as well as improving those already existing. There is no doubt that technological development has contributed to the revolution of information retrieval methods, especially in the area of managing connections between data. Some changes have impacted on electronic catalogues, allowing users to search for information via a larger number of access points than simply author name or title (as in alphabetical catalogues) or subject heading (as in subject catalogues). Depending on the capabilities of the system, we could search for information about a collection using such search terms as date, place or publication language. Electronic catalogues are able to quickly create a list of all the works by a given author in chronological order. However, it is still not possible to link information regarding differing ways of presenting the same title, e.g. in the form of text, film or theatrical adaptation. Further retrieval mechanisms in both traditional and OPAC catalogues have depended to a large extent on references or users being expected to initiate another search query. The FRBR (Functional Requirements for Bibliographic Records) idea has, obviously, been known within the librarian community for at least 20 years. So far, however, the technical limitations of library systems have not allowed the indication of relations between the various elements of a collection. Linked open data technology and the RDF (Resource Description and Framework) standard model have created a real opportunity to make changes in this area. New standards implemented for global Internet could be the perfect solution for problems facing libraries. Over the last decade the development of libraries has been, to a large extent, determined by ontologies, thesauri, classifications and other systems of knowledge organization. These tools organize the terminology used in online library catalogues, bibliographic databases and digital libraries, as well as relations between terms. For now, however, only a few solutions are providing high quality data to enrich