Administrators and teachers are concerned that English Language Learners (ELLs) in an urban elementary school in a southern part of the United States are not meeting required state standards in reading. Teachers have indicated that they do not always know how to differentiate instruction for ELLs. The purpose of this qualitative case study was to investigate teachers' implementation of differentiated instruction for ELLs. This research study was guided by the conceptual framework of Vygotsky's zone of proximal development and Tomlinson's theory of implementing differentiated instruction. The research questions investigated teachers' perceptions of differentiated instruction and their professional development for mediating instruction for ELLs. Six teachers from Grades 1 to 3 who teach ELLs with limited English-speaking ability and 2 ELL teachers participated in the study. Data were collected from transcribed interviews, open-ended surveys, and lesson plans. Inductive analysis was used to identify themes and commonalities within the collected data. The major themes included that differentiation is crucial for ELL instruction, teachers use varied instructional strategies to meet ELLs' individual needs, and teachers want purposeful and relevant professional development to meet ELLs' academic needs. The findings indicated a need for a professional development that include a systematic approach to differentiated instructional strategies to improve academic achievement for ELLs. A 3-day professional development was designed. The findings of this study and professional development may contribute to positive social change by increasing teachers' use of instructional strategies that align with the district's guidelines to improve learning and achievement for ELLs. [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.]