Within a suburban school system, an achievement gap exists since not all students are meeting state and national educational benchmarks. Despite the efforts of the school system, the achievement gap is endemic and persistent. To address the achievement gap, the school system instituted a Differentiated Instruction (DI) initiative. However, it was unknown how middle and high school teachers were implementing DI. The purpose of this study was to investigate the experiences and challenges of middle and high school teachers who implement DI. Vygotsky's constructivist theory provided the theoretical framework. Research questions explored the thoughts, attitudes, and beliefs teachers hold about DI as well as the challenges teachers face when incorporating DI. A case study methodology was used to investigate 10 middle and high school teachers' experiences with implementing DI through the use of interviews and a focus group. Analysis of data by identification of themes revealed that teachers believed that DI was a necessary component of teaching and increased student achievement, self-confidence, and success. However, teachers did not have enough training in DI, enough opportunities to collaborate, or enough time to investigate DI strategies. These results led to the development of professional training for teachers. The three-day training will provide teachers with content specific teaching strategies, will provide opportunities for the participants to collaborate with colleagues, and will provide time for the development of differentiated activities that can immediately be transferred to the classroom. Positive social change may result by helping teachers create learning environments that better foster student learning and achievement. [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.]