Less and less people use the OPACs. One of the possible answers of the libraries is to publish their data on the semantic web. The library OPACs are on the Internet, but they are – basically – standalone databases. Libraries - by their nature - like to cooperate. Z39.50, SRU, OAI-PMH are all interoperability standards making the cooperation possible between libraries. The semantic web makes the cooperation possible even between libraries and non library institutions. In my presentation I would like to explain the importance of the semantic web for libraries and I would also like to mention the presence of NSZL in the semantic web.