Over the last several decades, teacher education has emerged as a site of political struggle between diverse political and ideological camps seeking to shape the future of the teaching profession (Cochran-Smith, 2001). As the entry point into the teaching profession, teacher education programs have the potential to shape who becomes a teacher, how they are prepared to teach, and which knowledge informs the work of teaching. Although these policy debates have historically operated on the shared assumption that structural transformations to teacher education will transform the education system more broadly, the field has yet to systematically explore whether these structural transformations reach teacher identity and practice in how scholars and policymakers expect them to. In this three-paper dissertation, I leverage a sequential mixed-methods design to answer two central questions: (1) To what extent is teacher professional identity shaped by the broader reform contexts of teacher education? and (2) What role do teacher education programs play in mediating teachers’ professional identity formation? Across these analyses, I draw upon two separate sets of data: (1) qualitative data collected from across three teacher education programs based in the United States, including 73 interviews (with teacher candidates, teacher educators, and program leaders), 80 artifacts, and 75 hours of observations; and (2) an original survey administered to 950 in-service teachers across four states. Grounded in institutional theory, the three papers draw upon these data to investigate how reforms to teacher education become institutionalized and come to shape teacher education programs, as organizations, and teachers, as individuals. Together, this work provides the field with a conceptual framework and empirical tools to better understand the role of teachers in mediating broader reforms to teacher education, and the extent to which teacher education can be understood as a “policy problem” (Cochran-Smith, 2004).