This qualitative case study uses observations and interviews to examine the practice of differentiation by twelve collaborative middle school reading teachers in a school that has shown a decrease in the achievement gap for students with disabilities on end of year tests on state standards. Observations and interviews were analyzed to determine the teachers' knowledge and application of differentiated instruction. The results showed that four of the six teams possessed a comprehensive knowledge of the practice of differentiation and the data available to assist in planning lessons. Some differentiation was observed in each of the six classrooms. [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/en-US/products/dissertations/individuals.shtml.]