The purpose of this quasi-experimental, mixed method design study was to investigate the impact of Differentiated Instruction (DI) on student performance and achievement while implementing rigor in the Algebra II classroom. The call to increase rigor in high school classrooms increases daily, and educators are rethinking teaching and learning; the goal is to implement teaching models that are rigorous. DI strategies have been proven as pedagogical tools in high schools which have some positive effects on student learning outcomes and student achievement. Research in this study takes advantage of previous DI research studies to develop a theoretical framework suited for the investigation of student and teacher perceptions of DI, implemented with rigor by way of content, instruction, assessment, and the classroom atmosphere. The teacher-researcher wanted to determine whether there was a statistically significant difference in the means of the posttest scores of two classrooms; one with differentiated pedagogy, and the other with traditional pedagogy. To fulfill these purposes, the study tested the hypothesis utilizing an independent sample t-test. [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.]