Storage and transfer of information as well as interfaces for accessing this information have undergone a remarkable evolution. Nevertheless, information systems are still not ”intelligent” in the sense that they ”understand” the information they store, manipulate, and present to their users. A case in point is the world wide web and search engines allowing to access the vast amount of information available there. Web-pages are mostly written for human consumption and the mark-up provides only rendering information for textual and graphical information. Search engines are usually based on keyword search and often provide a huge number of answers, many of which are completely irrelevant, whereas some of the more interesting answers are not found. In contrast, the vision of a ”semantic web” aims for machine-understandable web resources, whose content can then be comprehended and processed both by automated tools, such as search engines, and by human users.