Recent studies support the use of differentiated instruction (DI) to improve literacy in content area classrooms. At the same time, research has found that few teachers implement DI purposefully or consistently. Accordingly, a case study design was used to explore middle school content area teachers' understanding and implementation of DI for content literacy at a site where it is an integral component of the response to intervention (RTI) process. The conceptual framework for this study was principles of differentiation, as defined and discussed by Tomlinson. Research questions were framed to examine how middle school content area teachers defined and implemented DI for content literacy by asking what they know, do, and need to effectively implement or sustain DI. Data were collected from a purposeful sample of 7 middle school content area teachers through semistructured interviews, a focus group, and unobtrusive data in the form of lesson plans. Descriptive and pattern coding were used to analyze the interview and focus group data for overarching themes. Emergent themes were validated through member checking, triangulated with themes identified in the lesson plans, and interpreted against principles of differentiation. Results indicated all participants were implementing DI for content literacy to some extent. The data also revealed participants wanted to improve their instruction but believed they needed additional supports: time to plan and gather resources, opportunities to collaborate with colleagues, and professional development to learn strategies to better differentiate for content literacy. The findings helped inform a project that provides identified supports for teachers as they differentiate instruction to improve content literacy, resulting in positive social change. [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.]