(Link) Physics Teaching Assistants (TAs) serve a critical role in supporting student learning in various classroom environments, including discussions and laboratories. As research-based instructional strategies become more widespread in these settings, the TA's role is expanding beyond simply presenting physics content to encompass facilitating student discussion and attending to student reasoning. At the same time, we recognize that these TAs are physics professionals and future faculty, and their teaching experiences in graduate school have the potential for long-term impact on their professional identities. Consequently, there is a need to enhance traditional forms of preparation to support TAs in this expanded role in ways that complement broader professional development opportunities. Enhancing TA preparation requires understanding how TAs make sense of their roles as instructors so that we may identify potential avenues for intervention that support the development of practices that are (1) supportive of curricular goals and (2) consistent with the TAs' overall pedagogical model. The intent of this thesis is to develop a single overarching framework for analyzing how TAs talk about and carry out their roles as instructors. We then apply this framework to a set of interview and video data from multiple semesters, and make claims regarding instances of coordination and dis-coordination between TAs' beliefs and practices. Furthermore, we are able to track changes in beliefs and practices along various time scales. Finally, we return to the issue of TA preparation by identifying features of enhanced professional and pedagogical development, drawn from results of these studies, that could operate within existing institutional structures.