This dissertation investigates the relationship between district supported teacher leadership and improving the teaching and learning of English learner (EL) students, a rapidly growing, yet continually underserved population of students. Utilizing a comparative case study design, this dissertation focuses on the work of two district-supported EL-focused teacher leaders within one district serving high populations of diverse EL students and the teachers with whom they work. The dissertation is presented as a set of three independent, but closely related, articles whose findings draw from a common empirical data source. The first article investigates the nature of the instructional leadership work of EL teacher leaders regarding the teaching and learning of EL students and the influence of this work on instructional change and educational opportunities for EL students district-wide. Drawing upon a distributed leadership framework centered on what I call the "English learner instructional core," this article focuses on how the two EL teacher leaders define and drive EL instructional change at multiple levels of the system through their "midlevel" positioning at the district and with schools and classrooms. The second article examines the work and discursive interactions between Robin, one of the focal EL teacher leaders and three focal grade level teams at varying levels of EL-related knowledge and experience. Drawing upon analyses of the teacher leader/teacher discourse, this article shows how each participant draws upon different discursive resources--such as research, Robin's EL-related expertise and experience, and the teachers' knowledge of their classroom and students--to negotiate EL-related professional learning in productive ways. The third article examines what EL-related knowledge and practices the focal mainstream classroom teachers took up through their work with the EL teacher leaders. Drawing from a framework based in social interchanges focused on problems of practice and relational agency, this article shows how work with the EL teacher leader on problems of EL instructional practice embedded in the teacher's classroom with her specific students and high levels of relational agency facilitated changes in the teacher's instructional practice. Together these three articles comprise a beginning understanding of how teacher leadership, whose work is focused on the teaching and learning of English learner students, may strengthen the educational opportunities for culturally and linguistically diverse children and youth. [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.]