This qualitative descriptive case study investigated the internal and external factors that support or limit teachers' decisions to adopt differentiated instructional approaches in the mathematics classrooms of a Southeastern United States Catholic elementary and middle school. School leaders and teachers face diverse student populations and challenges adapting instruction to meet student needs. Diverse student populations benefit from a range of enrichment and remedial tasks requiring more teacher attention to differentiating instruction. Teachers struggle to match appropriate resources and instructional plans with the specific needs of their students in math classrooms. The study site, representative in size and demographics of other area Catholic schools, featured a population of school administrators and math teachers from Grades 1-8. Qualitative data collection methods included teacher interviews, administrator interviews, and a focus group interview based on the central research question. The research subquestions aligned with the five attributes (trialability, complexity, compatibility, observability, and relative advantage) of the innovation component of Rogers' (2003) diffusion of innovations theory developed to explain how an innovation spreads among a social system. Findings confirmed differentiated instruction is viewed as a challenging yet critical component of Catholic school education. Factors such as school culture, professional development, and the availability of teaching assistants informed study participant decisions to differentiate. Findings will add to the literature and help inform school leaders in support of staffing, professional development planning, teacher and student retention, and the improvement of school culture. Future research questions emerged regarding differentiation practices in virtual and hybrid 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.]