With the emergence of social networking and Web 2.0 applications, libraries have the means to reach users through interactive Web-based tools patrons already use in their personal lives, such as Facebook and YouTube. In this study the authors aim to understand the ways that libraries are using YouTube for outreach purposes. Using a methodology adapted from studies in medical literature, the authors identified and analyzed library promotional videos on YouTube, both in relation to other works depicting libraries and librarians and as a unique category of content. In order to analyze the viewership of library promotional videos and its growth over time, view counts were compared at three points in time over a period of sixteen months. Using data made available by YouTube, the authors analyzed the top five referring Web sites to each video, thus allowing a basic understanding of the viewership of library promotional videos and their abilities to reach intended audiences. The authors also analyzed the many interactive features supported by YouTube to gain insight into the ways viewers were responding to and interacting with videos, including comments and the ability to mark videos as favorites. Finally, three examples of promotional videos created by libraries were selected as case studies. The creators of each video were interviewed about the creation processes and their insights into the effectiveness of their videos. A key finding was that while library professionals comprise a significant portion of viewers for library promotional videos, creators can increase viewership by the intended audience if they frequently and strategically feature online video content in Web sites, local or campus communication vehicles, and social media environments. The quantitative and qualitative measures developed for this study are offered as possible metrics for the assessment and evaluation of online library video content, and for libraries' use of social media forms. Based on these measures, and following the review of hundreds of videos with library-based content, the authors have derived a set of evidence-based best practices for the use of online video as a promotional tool by libraries. (Contains 4 figures and 7 tables.)