Positive relationships between administrators and school librarians were essential for increasing awareness of, and promoting advocacy for, school librarians who successfully fulfilled their six professional roles in contributing to school improvement and serving as additional educational leaders. The six professional roles of 21st century school librarians were that of instructional partner, technology leader, school improvement facilitator, collection manager, literacy leader, and advocate. To examine the perceptions of administrators and school librarians with regard to the professional roles, the researcher conducted a sequential mixed methods study of one public school district in Louisiana. The study consisted of two phases; the first phase included a pair of surveys sent to participating administrators and school librarians, prompting them to rate the importance and frequency of various tasks associated with the professional roles. The second phase included an interview protocol designed as a follow-up to build upon and complement the participants' feedback to the surveys. The data were analyzed for triangulation purposes and to answer the overarching research question: "To what extent are there differences between administrators' and school librarians' perceptions of school librarians' professional roles in school improvement in the TLK Public School System?" As a result of data collection and analyses, it was found that not only did administrators and school librarians share similarities in their perceptions with regard to the professional roles, but they also agreed on various reasons for identifying school librarians as additional educational leaders. The administrators and school librarians indicated that the various tasks and professional roles of school librarians were either important or highly important, and they were completed on a regular basis. The results of the study were generalized to the population, which was all administrators and school librarians employed in the public school system of study. The results were also used for educating other administrators, school librarians, and legislators about the professional roles of successful 21st century school librarians in contributing to school improvement and serving as additional educational leaders. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]