Students with specific learning disabilities (SLD) tend to have difficulty with reasoning and communication across many mathematical domains. The purpose of this study was, through open response questions, to examine the mathematics reasoning performance of 4 th grade students with SLD. Using the learner centered principles outlined by the American Psychological Association as the conceptual framework, this study investigated how students with SLD performed compared to their nondisabled peers and whether mathematical reasoning skills influenced their performances. "T"-tests were used to compare the performances of 500 students with SLD and 500 of their nondisabled peers in the 3 subskill areas of strategic competence, calculation accuracy, and communication clarity. "T"-tests were also used to investigate specific skills of mathematical domain strands of problem solving, fractions, decimals, estimation, and geometry. Analyses yield a significant difference in subskill areas and in specific mathematical domain strand skills between students with SLD and nondisabled students. Students with SLD demonstrated the greatest weaknesses in mathematical communication, multistep problem solving, and geometry; they demonstrated relative strengths in problem solving tasks involving addition, doubling, and multiplication with no regrouping. The findings indicate that intensive instructional interventions in mathematics are needed to remediate mathematical reasoning deficits of 4th grade students with SLD. The study contributes to social change by identifying the influence of reasoning on mathematical success so that instructional strategies can be developed to improve the reasoning skills, and therefore, test performances and subsequent graduation rates of students with SLD. [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.]