In recent years, tags have become a standard feature on a diverse range of sites on the Web, accompanying blog posts, photos, videos, and online news stories. Tags are descriptive terms attached to Internet resources. Despite the rapid adoption of tagging, how people use tags during the search process is not well understood. There is little empirical data on the use and perceptions of tags created by those other than the searcher. Previous research on tags focused on the motivations and behaviors of taggers, although non-taggers represent a larger proportion of Web users than taggers. This study examines how people use tags, created by others, during the search process. Forty-eight subjects were each assigned four search tasks in a within-subjects study. Subjects searched for text documents and images in a controlled laboratory setting, using information retrieval interfaces differing in their incorporation of tags. User behavior and perception data were collected through search logs and interviews. Both direct and indirect uses of tags across the search process were examined. Tags are used directly when they are clicked on, resulting in a new query, while tags are used indirectly when used for judgments of relevance or to obtain additional terms for query reformulation. Tags increased interactions with the information retrieval system, as subjects issued more queries and saw more search results when using the tagged interface. For both text and image searches, tags were used for query reformulation, predictive judgment, and evaluative judgment of relevance. Subjects interacted most frequently with tags on the search results page, using them for query reformulation and predictive judgment. Tags were more likely to be used for predictive judgment in text searches than in image searches. Subjects' understanding of tags focused on the role of tags in search, especially findability through a search engine. Tags were not uniformly perceived as being user-generated; site owners and automatic generation were mentioned as sources of tags. Several implications for the design of search interfaces and presentation of tags to support information interactions are discussed in the conclusion. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]