Graduate teaching assistants (TAs) are often responsible for teaching introductory courses to undergraduate science, technology, engineering, and mathematics students. The TAs are usually novices at teaching, and an important factor in their resilience and persistence in the face of inevitable challenges is self-efficacy. Little is known about what affects TA teacher efficacy or whether and how high- and low-efficacy TAs differ in their development as teachers. Bridging these gaps in the literature will inform best practices in developing and implementing professional development (PD) for TAs. Using a mixed-methods sequential exploratory research design, this study found differences in high- and low-efficacy TAs in both TAs' self-reflection and their students' perceptions. These differences concerned the focus of TAs' attention: inward at their own practices and emotions (salient in low-efficacy TAs) versus outward at the impact of their instructional guidance on their learners (prevalent in high-efficacy TAs). A proposed model of teacher efficacy based on TAs but generally applicable is presented to inform future research and provide suggestions for TA PD opportunities.