Despite the long-time call for change in mathematics instruction, there are several barriers to change, and instruction has remained stagnant overall, taking the form of traditional lecture. Instructional coaching has many of the attributes of effective professional development from the literature and could be a major support for instructional change. This study utilized an embedded mixed methods case study approach. The quantitative data was gathered using pre- and post-coaching scores using a theory-based tool; the qualitative data included coaching logs, transcriptions of coaching sessions, and interviews of the teacher. This study sought to answer the following research questions: (1) What impact does one-on-one content-focused coaching have on a middle school teacher's practice as demonstrated through her students' participation, specifically with regard to students communicating about the mathematics during a lesson? (2) What potentially productive activities or coaching conversations occur between the participating teacher and the coach during content-focused coaching that help the teacher improve practice? Results showed an increase for the teacher in the use of student communication structures around constructing arguments and critiquing the arguments of others both verbally and in writing, taking up feedback, and providing sentence stems and question prompts. Five key productive coaching moves were identified throughout the work including discussing content, naming practice, acknowledging effective teaching strategies to hone, brainstorming strategies, and creating action plans for lessons with the expectation of follow-up. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]