An achievement gap exists in mathematics between elementary school students with and without disabilities. Guided by the conceptual framework of constructivism, this project study addressed issues related to this achievement gap in the area of mathematics, specifically assessing the cause of this gap from the teachers' perspective. The research question was based on an investigation of the experiences of mathematics teachers who teach elementary students with disabilities in order to find what they perceived as the root cause of the achievement gap. This qualitative research design used case studies as the strategy of inquiry. Data were collected through interviews and observations of 6 teachers of students with disabilities. Interviews were conducted after observations with the participating teachers. Data were coded and analyzed first using typological analysis and followed by inductive analysis. The major finding of the study was that students with disabilities often lack basic skills and background knowledge for their school grade. Based on the findings, professional development training was created to help teachers gain knowledge about how to implement mastery learning within their classrooms and close achievement and skill gaps between students with and without disabilities. Implications for positive social change are related to the success of students with and without disabilities in learning mathematics. This professional development training project will help teachers differentiate their instruction by the use of student assessments. Assessments will allow teachers to focus on specific areas of student non mastery. Students will be able to achieve mastery of the learning goals before moving to other skills and learning goals. By becoming more proficient in their learning goals and having greater feelings of success, students will have greater confidence in their abilities to learn. [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.]