Physics Education researchers have been working to understanding how students learn physics, which has led to the creation of a body of research-based curricula. It is equally important to study novice instructors, graduate teaching assistants (TAs), who often teach these students. The study of TAs has similarities to how students have been studied: it is important to identify what preconceptions they often enter the classroom with, what resources they may have that they could apply to their physics teaching, and how both the classroom environment and past experiences affect what they are doing in the classroom. Although TAs are responsible for a significant portion of students' instruction at many universities, science TAs and their teaching have not been the focus of any significant amount of study.

This dissertation begins to fill this gap by examining physics graduate students who teach discussion sections for introductory courses using tutorials, which are guided worksheets completed by groups of students. While assisting students with their conceptual understanding of physics, TAs are also expected to convey classroom norms of constructing arguments and listening and responding to the reasoning of others. Physics graduate students enter into the role of tutorial TA having relative content expertise but minimal or no pedagogical expertise.

This analysis contends that considering the broader influences on TAs can account for TA behavior. Observations from two institutions (University of Colorado, Boulder and University of Maryland, College Park) show that TAs have different valuations (or buy-in) of the tutorials they teach, which have specific, identifiable consequences in the classroom. These differences can be explained by differences in the TAs' different teaching environments. Next, I examine cases of a behavior shared by three TAs, in which they focus on relatively superficial indicators of knowledge. Because the beliefs that underlie their teaching decisions vary, I argue that understanding and addressing the TAs individual beliefs will lead to more effective professional development. Lastly, this analysis advocates a new perspective on TA professional development: one in which TAs' ideas about teaching are taken to be interesting, plausible, and potentially productive.