Teacher feedback during the process of students' proving is important to consider because proving is often challenging for students, requiring feedback and support. Additionally, feedback has implications for authority and agency, which are constantly being negotiated. We examined the authority dynamics evidenced in a teacher's feedback actions while students proved geometry claims. By analyzing audio and video recordings of classroom proving discussions, we found that various teacher feedback types position either the teacher or students as authorities with regard to providing and validating mathematical ideas. We provide suggestions for research and practice with respect to teacher feedback and authority in proof instruction. [For the complete proceedings, see ED630060.]