Reform efforts and changes in mathematics education have brought on a shift towards a new vision of mathematics teaching in the United States. In light of recent accountability standards, the focus on teacher learning within the context of mathematics professional development is even more pressing. Prior research on teacher learning in the context of professional development has focused on how different teacher characteristics influence teacher learning, such as teaching experience, content knowledge, beliefs, and attitudes. Few studies have considered the role of mathematics teacher identity in relation to teacher learning and participation in professional development. In this dissertation, I explored the relationship between teachers' mathematics teacher identity and their experiences and participation within professional learning communities. Specifically, this study examined the construct of mathematics teacher identity in relation to teacher tensions and negotiations of tensions as they participated in mathematics professional development. Additionally, this study drew on the experiences of two novice teachers as they transitioned into new professional learning communities with respect to their mathematics teacher identities. Data were drawn from semi-structured interviews, observations of professional learning community meetings, and teacher questionnaires collected over the course of one academic school year. Findings from this study highlight the importance of attending to mathematics teacher identity in relation to teacher learning in professional development. Specifically, mathematics teacher identity was found to play a role in the ways teachers learned, participated, and were positioned by others in their professional learning communities. In light of these findings, I propose the need for teacher educators and professional development programs to recognize the lived experiences and personal narratives that inform the mathematics teacher identity as well as to factor in how community shapes identity and teacher development when designing professional development experiences. [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.]