▼ Social bookmarking and other Web sites allow users to submit items and tag them with keywords, creating a folksonomy. Little empirical research has been done on folksonomies. This study compares the search information retrieval performance of folksonomies against search engines and subject directories. Thirty-four participants created 103 queries with various information needs. Results from each IR system were collected and participants to judged relevancy. Folksonomy search results overlapped with those from the other systems, and URLs returned by multiple systems were more likely to be relevant. URLs found by both search engines and folksonomies were significantly more likely to be judged relevant than those returned by any single IR system type. The search engines had the highest precision and recall. The directories usually performed better than the folksonomies in precision, though Del.icio.us was indistinguishable from the directories. Folksonomies and directories had similar recall scores. Folksonomies are promising, and may be able to improve Web search performance. Advisors/Committee Members: Robins, David B.