In an era of new standards and emerging accountability systems, an understanding of the supports needed to aid teachers and students in making necessary transitions in mathematics teaching and learning is critical. Given the established research base demonstrating the importance of justification and reasoning in students' mathematics learning and the heightened emphasis on students' abilities to demonstrate the mathematical practices outlined by the Common Core State Standards Mathematics Practice Standards, this study is timely in that it examined mathematical argumentation as it is currently enacted in today's classrooms. The study investigated students' mathematical argumentation as it is currently practiced in high school classrooms to understand the ways in which the teacher education and professional development communities may better support teachers in this new era of standards and accountability. Five high school Algebra I teachers and their classes comprised the sample. Using a multiple case study design, data in the forms of classroom observations, teacher interviews, and detailed field notes were collected and analyzed. The within- and cross-case analyses revealed a modest number of episodes of mathematical argumentation with a primary focus on using definitions, properties, and procedures to establish students' claims. Further, teachers fostered mathematical argumentation in their classrooms for a variety of reasons, many of which focused on factors affecting learning not explicitly supporting the learning of new mathematical ideas. Findings suggest that teachers may view argumentation as a means of assessing students' knowledge rather than as a mechanism of learning. [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.]