Over the last several decades, language teacher identity has become a growing field of research due to its link with teacher performance and student achievement (Beauchamp & Thomas, 2009; Beijaard et al., 2000). However, graduate teaching assistants (GTAs) are often excluded from the LTI research despite language departments' increasing reliance on their "integral labor" (Robertson & Yazan, p. 10). The three studies comprising this dissertation examined the professional identity and agency construction of GTAs in additional languages departments in the US, with a focus on participants' "willingness," "capability," and "power" (WCP) to position themselves as teachers. Study 1 used positioning analysis to investigate how GTAs position themselves in their narratives of teaching experiences, revealing that their professional identities are closely related to their professional goals, with varying levels of "willingness," "capability," and "power" to embody desired positions. The study also found that imposed positions or authority-mediated positioning often conflict with the GTAs' self-positioning, leading to tensions and limitations in their identity construction. The implications of these findings suggest the need for supportive and empowering contexts for GTA identity development. Study 2 explored the effects of a training module on GTAs engaged in language teaching materials development. Findings suggested that the training module had a positive effect on both novice and experienced groups, with the novice group showing increased perceptions of "capability" and the experienced group showing increased "willingness" to engage in materials development. Both groups also demonstrated a re-evaluation of their perceived "power" as materials developers, which had reciprocal effects on their perceptions of capability and willingness. The implications of these findings highlight the importance of providing GTAs with professional development opportunities, particularly in areas such as materials development that are directly applicable to their teaching contexts. Study 3 examined the role of modal periphrases in GTAs' positioning as (in)agentive teachers, based on the positioning theory framework of WCP. The study found that the modal periphrases of "can" and "have to" are particularly significant in this regard, and that the deontic properties of these modal constructs have important consequences for the speaker's perceived WCP. The study also highlighted the role of internalized norms and perceived support in mitigating conflicts between sources of obligation. The implications of these findings emphasize the importance of understanding how language shapes agency in educational contexts. Overall, these studies provide insight into the complexities and challenges of GTA identity construction and highlight the need for supportive and empowering contexts for GTA professional development. They suggest potential strategies for maximizing affordances and minimizing barriers to GTA identity development, including emphasizing GTAs' positioning as language teachers and providing adequate support and training. These findings have implications for language departments, TA supervisors, program directors, and all entities invested in the professional development of graduate students as teachers. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]