Many undergraduate students will experience being taught by graduate teaching assistants (GTAs); therefore, focusing on GTA pedagogy is in the best interest of the students and university as a whole. Based on a Masters of Arts in Teaching program, a 5-week intensive summer training session for science, technology, engineering, and mathematics (STEM) GTAs was created by an interdisciplinary group of faculty. This study investigates how the intensive summer program can improve STEM GTA pedagogy by revealing STEM GTAs' beliefs about their role as a teaching assistant. Understanding what GTAs believe about their role will provide insight as to how to better prepare GTAs and, hence, help to improve STEM undergraduate education. The results of the study showed that GTAs' beliefs on teaching were initially more narrow in scope and then shifted to a greater understanding of the role of "teacher" in the classroom. Additionally, skills that helped GTAs with their pedagogy reportedly helped the GTAs as graduate students as well. These findings are insightful as we continue to learn how to prepare and train STEM GTAs.