Informed by Critical Theory and Poststructuralist Theory and the intersections of agency, power, ideology, discourse, capital, and language, this study investigates how three ITAs construct their professional identities as instructors at a U.S. university. To gain an in-depth understanding of ITAs' professional identities development, the researcher uses a qualitative approach with a multi-case study design to examine various data and variables including a) undergraduates' feedback to ITAs' instruction, b) ITAs' English language use in academic settings and its influence on their teaching, and c) the role of ITAs' supervisors on their professional practice. Through narrative analysis, the researcher analyzes data from interviews, classroom observations, and research journals. Findings suggest that both course evaluations and ITAs' interactions with supervisors and colleagues influence their professional identity formation. Additionally, the ITAs' English language use in academic settings reflects their desire of becoming native English speakers for an audience of mostly US undergraduates.