Social tagging ranges among the ``killer applications'' of Web 2.0. An ever-growing international community uses Web sites such as the photo database Flickr and the bookmarking service Delicious. In addition, a number of other portals use tagging to compile user-specific metadata on information on any subject—whether it be travel destinations, personal contacts, films, or museum exhibits. Retrieving and storing information via tagging seems to meet users' needs for a number of purposes and in many contexts. Starting with a synopsis of the current literature on social tagging and then focusing on the results of two surveys—qualitative interviews and an online questionnaire—this article explores the potential and limitations of tagging as a tool for organizing shared and personal knowledge.