The purpose of this dissertation is to investigate the discursive process of the negotiation and construction of teacher identity in a US-based Master of Arts for Teachers of English to Speakers of Other Languages (MATESOL) program. This study explores how both native-English-speaking (NES) and non-native-English-speaking (NNES) student teachers' identities as English language teaching (ELT) professionals are shaped by professional discourses in the program, and how their (non)native status influences this enculturation process and their teaching practices. While teacher identity has been an emerging subject in research on language instruction over the last decade, the NNEST/NEST dichotomy remains the most prevalent way of theorizing teacher identity in TESOL (Menard-Warwick, 2008), and the process of the search for and construction of a professional teacher identity, especially within MATESOL programs, is understudied in the field of TESOL and Applied Linguistics. This longitudinal ethnographic study begins to fill this gap by examining case studies of four student teachers: one international graduate student from China, one immigrant from Afghanistan, and two native speakers of English from Northern California. Ethnographic data were collected through both participant- and nonparticipant-observation over one academic year. In light of the post-structural view that language and identity are mutually constitutive, analysis was guided by Bakhtin's (1981) "dialogism" and "heteroglossia" and Lave and Wenger's (1991) and Wenger's (1998) "Community of Practice" to reflect on power and agency in professional identity development. Textual analysis of the discourses drawn by the participants was carried out by using Systemic Functional Linguistics (SFL) (Halliday, 1994/2004) and Appraisal (Martin & White, 2005). This study reveals how curricular design, practicum requirements, and the selected theories and readings that represent the various authoritative discourses in TESOL have influenced and guided the developing identity of the student teachers. However, the study also finds that discourses are not equally available to all participants to negotiate spaces in the program and in the broader TESOL community. Their negotiation and construction of an identity as an ELT professional was influenced and shaped by, to varying degrees, the education and experience they brought with them to the program, personal beliefs and identities, their motivation and purposes for becoming teachers, their cultural and language backgrounds, their gender identity in their families and society, their relationships with other members in the local community, and their relationships with authoritative discourses. This study offers important implications for teacher professional development and teacher preparation programs. [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.]