Purpose: This qualitative case study explored the best practices of differentiation of Tier 1 instruction within a multi-tiered system of support for English Language Learners who were predominately Spanish speaking. Theoretical Framework: The zone of proximal development theory, cognitive theory, and the affective filter hypothesis guided this study. Methodology: A case study approach was used to address three research questions. Data was gathered in the form of interviews and documents. Participants responded to 12 open-ended questions. A total of 10 participants were engaged in the study. The sample population consisted of five participants from a private school and five participants from a public school. Both schools were located in Southern California. Findings: An examination of qualitative data from one public and one private school indicated that both schools had a strong understanding of the multi-tiered system of support. In addition, a close look at the findings showed teachers applied a version of the problem-solving cycle, in which they incorporated a reflective component, to align practices and resources. Conclusion: The findings support the need to find alternative approaches to differentiating or modifying a lesson's task and environment to support English Language Learners. In addition, there is a need to find additional teaching methods that encompass all four approaches to differentiating instruction and to see how effective they are in supporting English Language Learners and helping them to become college and career ready. Recommendations: The study was limited to 10 participants and, thus, to their instructional practices and knowledge of methodology in helping English Language Learners acquire a new language and new learnings. Future studies could replicate the study with a larger sample size that includes various school districts or various schools within a district. This would inform educators on professional development topics and research needed to support English Language Learners. [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.]