Teacher accountability and test scores currently drive many practices within the field of education. At the same time, student populations have become increasingly diverse, causing classroom teachers to feel pressured to teach to the test. Differentiated instruction has emerged as a best practice to help maximize learning for all students. However, the definition and implementation of the instructional approach remains inconsistent. Guided by the concepts of Vygotsky's zone of proximal development and Tomlinson's differentiated instruction, this study examined the connection between these 2 concepts as they relate to teachers' perceptions of differentiated instruction in the content area of reading. This research case study was conducted with fourth and fifth grade general education teachers in an urban, upstate Illinois school district, where student needs are highly diverse, to discover teacher perceptions of differentiated instruction. The research questions focused on the perceptions of teachers as to how they define, implement, and what they need to differentiate reading instruction successfully. Interviews and classroom observations were conducted with fourth and fifth grade teacher participants. Narrative data from the five teacher participants showed a common representation of what teachers "DO" in the classroom during instruction but how teachers define and implement differentiated instruction is inconsistent. Major themes found within the data address differentiated instruction that involved a) use of assessments to determine reading levels, b) guided reading instruction in small groups, and c) customized reading instruction to meet each student's reading needs. Additional insight gained from the study about what is needed to implement differentiated instruction included more time, flexibility in scheduling, smaller class sizes, appropriate resources, and a teaching assistant. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]