In this research study, an instructional intervention protocol is described. This intervention was designed to help teachers to engage English Learner students (ELs) in mathematical problem-solving lessons and reasoning through discourse. The "Discursive Mathematics Protocol" (DMP) integrates Polya's classic problem-solving framework and incorporates research-based instructional strategies that are specifically intended to benefit ELs. These research-based strategies are grounded in theories of academic language development, which suggest that ELs learn mathematics knowledge through negotiating meanings, communicating ideas, and engaging in mathematical discourse. A qualitative descriptive case study was conducted to examine how the DMP, when used during problem-solving lessons, supports the development of ELs' mathematics register as well as how the teacher focused on developing the mathematics register of her ELs. The mathematics register is a set of communication styles, characteristics, modes of arguing, and meanings associated with the language of mathematics. The primary focus of this study was to explore how the implementation of the DMP helped a fifth-grade teacher and her ELs at an urban elementary school to develop the mathematics register. Findings in this study provide insights into how the implementation of the DMP supported the teacher to focus her instruction on supporting her ELs to advance their mathematics register, English language skills, and mathematics knowledge. The DMP also helped the teacher to engage her ELs in meaningful mathematical discourse while they solved worthwhile tasks. Implementation of the DMP allowed students to enhance their oral and written communication skills. Incorporating the DMP to plan and teach mathematics to advance the mathematics register is recommended. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]