Folksonomies (‘bottom-up’, user-generated categorisations) are an integral part of ‘Web 2.0’ technologies, and have been employed in a wide variety of online contexts, including businesses (such as Amazon), social networking sites, photo and music sharing, blogs, and OPAC ‘discovery layers’. These unstructured knowledge organisation systems will also have an important role in future Semantic Web ontologies. Despite many shortcomings, the use of tags and folksonomies has become standard practice in indexing the Social Web. This conceptual paper poses four questions: 1. How have folksonomies been incorporated into libraries? 2. Does the use of tag clouds and folksonomies add value to the user’s experience of the library catalogue? 3. Is it possible for folksonomies to supplant traditional ‘top-down’ taxonomies? 4. What implications does the use of folksonomies have for the work of librarians? These are addressed through reference to the literature, as well as surveying examples of folksonomies in library catalogues.