Lue, KristynThe field of higher education has been concerned with the retention of underrepresented students of Color in the science, technology, engineering, and math (STEM) fields over the last few decades. STEM identity development has emerged as a useful analytic framework in this research, as students with stronger STEM identities—students who recognize themselves and are recognized by others as “STEM people”—are more likely to persist in the STEM fields. STEM identity develops through the process of socialization, in which agents of socialization set and maintain the norms, culture, and values that newcomers in the STEM fields should emulate. At institutions of higher education, instructors act as primary agents of socialization, signaling who “belongs”—and who doesn’t—in the STEM fields. Although prior research has identified the ways in which mathematics courses gatekeep underrepresented undergraduate students of Color out of the STEM fields, little research has focused specifically on undergraduate mathematics socialization. Furthermore, the role of graduate teaching assistants (GTAs) as agents of mathematics socialization remains unexamined, despite the large role they play in teaching lower-level undergraduate mathematics courses. This qualitative dissertation, which is grounded in Critical Race Theory and Critical Whiteness studies, utilizes critical ethnographic methods in order to examine the ways in which GTAs at a historically white [college and] university (HWCU) serve as agents of mathematics socialization for underrepresented undergraduate students of Color. Through fieldwork, individual interviews, and a series of focus groups with ten GTAs at a large, public, research-1 institution in the Mid-Atlantic region of the United States (MAU), this dissertation study explored: (1) GTAs’ perceptions of the dominant culture (e.g. values and practices) of the mathematics department at their institution, and whether they sought to align with or diverge from this culture, (2) the opportunities and constraints GTAs faced in breaking from these normative values and practices, and (3) whether the ways in which GTAs described trying to break from these practices reinforced the systematic exclusion of underrepresented undergraduate students of Color in their mathematics department. Key findings include four major themes: a culture of individualism and the hidden necessity of social ties in the mathematics department at MAU, the valuation of teaching as a means of doing research, attempts by GTAs to shift normative narratives of mathematical success, and identity tensions in supporting underrepresented undergraduate students of Color. These findings highlight the importance for agents of mathematics socialization to explicitly center race, racism, and power when trying to be inclusive of underrepresented undergraduate students of Color in university mathematics settings. Without doing so, racism and whiteness are reproduced and perpetuated in the mathematics socialization of underrepresented undergraduate students of Color, despite good intentions.enDissertationMathematics educationHigher educationEducational sociologyCritical Race TheoryCritical Whiteness StudiesGraduate Teaching AssistantsMathematics Socialization