For the inclusive education indicated in the Individuals with Disabilities Education Act (IDEA) to occur at the high school level, teachers must be prepared to teach students with multiple ability levels and learning profiles in one classroom. Since the passage of IDEA, large numbers of students with mild to severe learning needs have been placed in inclusionary regular education classrooms. Differentiated Instruction (DI) is one method that assists teachers in accomplishing inclusion. The specific problem addressed in this study is teachers are not effectively mentored or motivated in the application of DI within their classroom curriculum. The purpose of this qualitative multiple case study was to explore the motivation, mentoring process, and professional development underlying successful high school single subject teachers who apply differentiated instructional strategies that promote the inclusion of students with disabilities. The population for this study was high school teachers from suburban Denver schools that require DI and assess teacher effectiveness in DI, during teacher evaluations. Schools representing a variety of socio-economic backgrounds and practice, and support the inclusion of students with disabilities in their typical classrooms were purposively chosen. Teachers at the study sites were those who practice key elements of DI such as assessment based instruction, authentic assessment, flexible learning groups, and multiple methods of active instruction. The study focused on teachers who apply DI while including students with disabilities. Four main themes materialized from the interviews concerning Participants' views: 1) participants' viewed themselves as facilitators; 2) participants' valued strong relationships with students; 3) participants' maintained high expectations; and 4) participants' practiced continuous differentiation of instruction. Participants' described their training in DI as minimal. They viewed collaboration with colleagues and personal reflection as essential ingredients in their success in DI. [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.]