The vision of the Semantic Web (SW) is gradually unfolding and taking shape through a web of linked data, a part of which is built by capturing semantics stored in existing knowledge organization systems (KOS), subject metadata and resource metadata. The content of vast bibliographic collections is currently categorized by some widely used bibliographic classification and we may soon see them being mined for information and linked in a meaningful way across the Web. Bibliographic classifications are designed for knowledge mediation which offers both a rich terminology and different ways in which concepts can be categorized and related to each other in the universe of knowledge. From 1990-2010 they have been used in various resource discovery services on the Web and continue to be used to support information integration in a number of international digital library projects. In this chapter we will revisit some of the ways in which universal classifications, as language independent concept schemes, can assist humans and computers in structuring and presenting information and formulating queries. Most importantly, we highlight issues important to understanding bibliographic classifications, both in terms of their unused potential and technical limitations. Classification revisited: a web of knowledge