In an urban Title 1 school district, the average number of Grade 3-5 students who scored proficient or advanced on the state standardized assessment was 37.3% below Grade 3-5 students countywide and 19.4% below Grade 3-5 students statewide. Low mathematics scores may indicate a gap in practice that affects student achievement. The purpose of this descriptive case study was to examine teachers' descriptions of instructional strategies implemented to mediate instruction for students who struggle in mathematics. This study was based on the conceptual framework of Tomlinson's differentiated instruction (DI), a means of accommodating the varied ways that students learn. The research questions guided an inquiry into how teachers of students in Grades 3-5 in the school district described DI in practice and explained their professional development on DI strategies. Data were collected from individual interviews with 8 elementary school teachers of students in Grades 3-5 and an instructional coach in the local school district with 3 or more years of experience who considered themselves knowledgeable of DI. I coded the meaningful data collected from the interviews and subsequently formed themes. Themes that emerged from the interviews included defining DI, tools for instruction, classroom set-up and transitioning, assessments, professional development, and grouping strategies. Results revealed that interviewees were confident in defining and facilitating DI grouping strategies for English and language arts but felt the need for more professional development to implement and understand DI in relation to mathematics. The study outcome may impact social change by affecting the gap in practice through professional development that helps teachers implement DI strategies in the classroom to improve student achievement in mathematics. [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.]