The purpose of this dissertation is to develop an instrument that can measure teacher beliefs about mathematical discourse as a continuum by exploring and describing qualitatively distinct levels of teacher beliefs, namely, Univocal, Partial Univocal, Emerging Dialogical, and Dialogical. Prior research indicates the importance of understanding teachers' beliefs in the development of their teaching practice and the impact of their teaching practice in K-12 classrooms. However, assessing teacher beliefs has been difficult and often unsuccessful largely due to poor conceptualizations and measurement challenges associated with assessing beliefs. The field of teacher education is in need of carefully conceptualized and operationalized measures of teacher beliefs that are valid and reliable to understand relationships among teacher beliefs, teaching practice and student outcomes in order to improve instructional practice and inform educational policy. This study reports the development of such a measure using the four building blocks recommended by Wilson (2005). The item design consisted of hypothetical teaching situations that present students' correct and incorrect thinking, and to then ask the teacher to respond in order to lead a mathematical discussion. The participants in the study include a total of 168 pre-service teachers in 10 teacher education programs in the California State University system, the University of California system, and one private university, and 27 in-service teachers across the states. Results showed generally positive evidence for the validity and reliability of the measure. In addition, the findings of this dissertation suggest that the teacher belief about mathematical discourse (BMD) measure is a potentially promising tool for informing and designing elementary mathematics method courses. Suggestions for further research on the validity and reliability evidence are outlined. [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.]