To ensure that our students can compete in a global economy, successful teaching and learning of mathematics must become a priority within our schools. However, schools continue to track or group students by ability, which, in turn, systematically leaves students behind. Detracking will create a school system that provides both equity and excellence in mathematics education. Detracking encompasses more than just minor curriculum or school structure adjustment. Done correctly, detracking takes time with specific attention to both the students' and teachers' needs. Teacher learning is an integral piece to ensure that our students receive the full benefits of detracking. Professional development must be "curriculum based, intensive, and sustained over time with follow-up support" (Burris & Garrity, 2008, p. 98). Teachers will require professional development to assist them in replacing traditional practices with differentiated instructional techniques. This study analyzed the impact of a detracked mathematics program and teachers' perceptions of the implementation of differentiated mathematics instruction. This mixed-methods study's data was analyzed both qualitatively and quantitatively. Student data, final mathematics course grades, and PSAT scores were collected via the district data system to compare the academic achievement of tracked and detracked students. The qualitative data consists of teacher interviews, observations, and lesson plan reviews designed to gather data on teachers' perceptions within the implementation of differentiated instruction. The quantitative data indicated that there was no negative impact on the detracked students' performance on PSAT mathematics scores. However, the differences within the range of PSAT scores of the detracked students indicated that the students had a wide variety of mathematics ability. The qualitative data collected in this research study resulted in three major categories which included Differentiated Instruction, Teacher Experience, and Professional Development. Through analysis of these categories, teacher lesson plans, and teacher observations, it was found that teachers are attempting to implement differentiated instruction activities within their lessons; however, teachers indicated that they require additional training and support on how to implement differentiated instruction within their mathematics classes. The variety of student ability in a detracked mathematics course demands that teachers understand how to develop lessons that address the needs of all their students. It is essential that professional development be afforded to teachers prior to the detracking of students, as well as throughout the school year, to inform teachers, involve application and collaboration, and support teachers to ensure that teachers reach the growing population of diverse learners. [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.]