According to the National Council of Teachers of Mathematics (NCTM) standard of communication, ??Instructional programs from pre-kindergarten through grade 12 should enable all students to...communicate their mathematical thinking coherently and clearly to peers, teachers, and others?? and students need to learn ??what is acceptable as evidence in mathematics?? (NCTM, 2000, p. 60). But do teachers have a clear understanding of what is acceptable or do they believe that the only acceptable explanations are the ones that they themselves gave to the students? Can teachers accept alternative forms of explanation and methods of solution as mathematically accurate or do they want students to simply restate the teachers?? understandings of mathematics and the problem? The focus of this dissertation is the authoritative discourse practices of classroom teachers as they relate to individual students and large and small groups of students. In this case study, I examine the interactions in one eighth-grade mathematics classroom and the possible sharing of mathematical authority and development of mathematical agency that take place via the teacher??s uses of authoritative discourse. A guiding objective of this research was to examine the ways a teacher??s discursive practices were aligned with her pedagogical intentions. The teacher for this study was an experienced eighth-grade mathematics teacher at a rural Central Texas middle school. The teacher was a participant in the Middle School Mathematics Project at Texas A&M University. Results of an analysis of the discourse of six selected classes were combined with interview and observation data and curriculum materials to inform the research questions. I found that through the teacher??s regular use of authoritative discursive devices, mathematical authority was infrequently shared. Also the teacher??s uses of authoritative discourse helped create an environment where mathematical agency was not encouraged or supported. The teacher??s use of various discursive devices helped establish and maintain a hierarchy of mathematical authority with students at the lowest level reliant on others for various mathematical decisions.