The purpose of this qualitative phenomenological study was to examine the perceptions of middle school teachers regarding differentiation of instruction for students with mild to moderate disabilities. Although there has been little documentation in the literature regarding instructional differentiation five factors that facilitate change have been identified in the framework. Data collection strategies included one-on-one semi-structured interviews and questionnaires. Analysis of data occurred in three phases: (a) transcribing interviews, (b) member checking of the transcripts, (c) coding-recoding of the data. The analysis of data was based on the theoretical proposition that student achievement, positive behaviors, and increased learner motivation fosters the change process when differentiation is implemented. The credibility of the analysis was protected by triangulation of data through multiple sources of evidence, code-recode strategy, and member checking. The results indicate that differentiation has a positive influence on student learning. Five themes emerged from the analysis of data: (1) teachers' personal definition of differentiation, (2) a variety of implementation strategies yield positive student results, (3) barriers to implementation of differentiation, (4) needed supports for implementation of differentiation strategies, and (5) student 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.]