The purpose of this qualitative research study was to understand teachers' perceptions of differentiated instruction and the rationale behind their resistance to implementing this strategy on a regular and consistent basis in an elementary school in the state of Maryland. Based on data collected from the state assessment in mathematics in Grades 3 to 5, students did not make significant academic gains compared to some other schools in the county. Data was collected through interviews and classroom observations in order to gain an understanding of teachers' perceptions of differentiated instruction and their resistance to its implementation. The research methodology was a phenomenological design, which most accurately revealed the participants' lived experiences of the phenomenon being studied. A purposeful sampling allowed this inquiry to focus on mathematics teachers. The sample consisted of four participants, who taught mathematics at the elementary level. Findings of the study revealed that the participants had a fair understanding of differentiated instruction and its benefits in aiding students' achievement. Participants revealed that the lack of professional development in differentiated instruction was one of the main barriers to their regular and consistent implementation of the technique. Additionally, the participants shared that the lack of materials, pacing through the curriculum, as well as lack of administrative support hindered their ability to effectively implement differentiated instruction. Recommendations include that teachers be provided with ongoing professional development in differentiated instruction by their administration and consistently supported if they are to effectively meet the needs of all 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.]