Recent mathematics education reforms (NCTM, 2000) have resulted in increased opportunities for teachers to learn new teaching practices. However, the relationship between teacher professional development and the actual implementation of new practices is unclear. I posit that a teachers' decision to implement newly learned practices is strongly connected to her identity. However, the relationship between experienced teachers' implementing of new practices and their ways of identifying has been largely ignored. How teachers learn to implement mathematics pedagogies that promote equitable student outcomes is a particular concern. Complex Instruction (CI), one such pedagogy, is a set of principles and practices for organizing cooperative group work in heterogeneous classrooms (Cohen, 1994; Cohen & Lotan, 1997). In this dissertation, I report on four elementary teachers who chose to implement CI. Building on theories that link learning, practice, and identifying (e.g. Wenger, 1989), I analyze the identifying stories of the teachers using tools from narrative analysis (e.g. Gee, 1991; Labov, 1972) in order to understand how identifying and change in teaching practice impact each other. I elicited narratives in three contexts: individual interviews about CI lessons, group discussions, and presentations about CI to other teachers. This study explores how teachers come to identify as new kinds of teachers while implementing new practices and how those ways of identifying become reasons for the teachers to continue those practices. Teachers identified as CI mathematics teachers by reifying narratives of their CI mathematics teaching experiences and integrating them into existing identifying narratives or negotiating between new narratives and existing narratives. This study provides a language for the difficulties inherent in teacher learning (i.e., implementation of new practices) that is respectful of teacher knowledge and experience. The explication of the impact of mathematics education reforms on teachers' lives and practice has implications for policy and professional development. [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.]