Since the adoption of faceted search in a small number of academic libraries in 2006, faceted library catalogs have gained popularity in many academic and public libraries. This dissertation seeks to understand whether faceted search improves the interactions between searchers and library catalogs and to understand ways that facets are used in different library environments. Interactions under investigation include possible search actions, search performance, and user satisfaction. Faceted catalogs from two libraries, the University of North Carolina at Chapel Hill (UNC-CH) Library and the Phoenix Public Library, are chosen as examples of two different facet implementations. To observe searchers in natural situations, two log data sets with over 3 million useful records were collected from the two libraries' servers. Logs were parsed, statistically analyzed, and visualized to gain a general understanding of the usage of these faceted catalogs. Two user experiments were conducted to further understand contextual information, such as the searchers' underlying motivations and their perceptions. Forty subjects were recruited to search different tasks using the two different catalogs. The results indicate that most searchers were able to understand the concept of facets naturally and easily. Compared to text searches, however, faceted searches were complementary and supplemental, and used only by a small group of searchers. When browsing facets were incorporated into the search, facet uptake greatly increased. The faceted catalog was not able to shorten the search time but was able to improve the search accuracy. Facets were used more for open-ended tasks and difficult tasks that require more effort to learn, investigate, and explore. Based on observation, facets support searches primarily in five ways. Compared to the UNC-CH Library facets, the Phoenix Library facets are not as helpful for narrowing the search due to both its essential and lightweight facet design. Searchers preferred the Book Industry Standards and Communications (BISAC) subject headings for browsing the collection and specifying genre, and the Library of Congress subject headings (LCSH) for narrowing topics. Overall, the results weave a detailed `story' about the ways people use facets and ways that facets help people employ library catalogs. The results of this research can be used to propose or refine a set of practical design guidelines for designing faceted library catalogs. The guidelines are intended to inform librarians and library information technology (IT) staff to improve the effectiveness of the catalogs to help people find information they need more efficiently.