The purpose of this qualitative study was to gain an understanding of how a principal's English Language Learner (ELL) educational experience and professional development impacts the collective efficacy of a school staff with a high ELL population. Research questions included the role of principals' ELL preparation, their history of professional development with an ELL focus, the shared leadership that takes place in their buildings, and how their leadership in general plays a role in the collective efficacy that impacts ELL success. Principals leading a school where at least 25% of the population was ELLs were chosen for this research work. These principals were interviewed to gain a better understanding of their educational and professional development background with ELLs. To understand the impact that principal leadership had on collective efficacy within the school, teachers, who were instructors of ELLs in their classrooms, were chosen to go through the interview protocols. The semi-structured interviews were conducted with 12 participants from three different schools. Themes included that pre-service ELL courses would be beneficial to teach and lead schools of ELLs, principals and teachers play a critical role in creating an environment of shared leadership for ELL success, and ELL values and beliefs foster collective efficacy in a school. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]