The current literature indicates a need for quality studies on how best to close the achievement gap between elementary English language learners (ELLs) and their English speaking peers. The purpose of this sequential explanatory mixed methods research study was to determine the relationship between standards-based differentiated instructional (DI) strategies and the literacy achievement of ELLs and how knowledgeable are teachers in implementing differentiated instruction to support ELLs. Quantitative Phase One involved collecting and analyzing data from these sources: (a) literacy ELL Georgia Criterion-Referenced Competency test data for grades 3, 4, and 5, from 2004 through 2009, (b) data from teacher observation, and (c) self-reported responses from a teacher survey. Qualitative Phase Two involved a focus group discussion and an examination of data collected from Phase One that was subsequently coded into themes surrounding differentiated instructional practices. The setting for the study was a culturally diverse southeastern metropolitan public school district. The population consisted of the NCLB Adequate Yearly Progress ELL subgroup from five Title I elementary schools in grades 3,4, and 5 and grades 3, 4, and 5 teachers trained in differentiated instruction that worked with ELLs during the intervention years. The quantitative student performance data showed that DI intervention significantly affected the literacy achievement of ELLs. The qualitative data provided voice to additional data collected from the quantitative teacher survey and classroom observations. The benefits of the study inform (a) teachers, (b) local district and school administrators, and (c) policymakers about the how of implementing differentiated instruction, especially as states choose to adopt the new Common Core State Standards Initiatives. [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.]