Wondering whether they are really making a difference to young people's mathematics learning is a question that most teachers have probably wrestled with at some stage of their careers. However, evidence from a multitude of research studies shows that students' mathematics learning and their dispositions towards mathematics are indeed influenced--for better or for worse--by the teaching that they experience at school (see Mewborn, 2003, for a review of this research). In other words, teachers "do" matter. It is difficult for researchers to specify exactly how different types of teaching and teacher qualities affect student achievement because this would require untangling the complicated relationships that exist between teacher characteristics, teaching practices, and student learning. Nevertheless, the general trends in these relationships are clear. In this article, the author illustrates some of these trends by drawing on her experiences in working with pre-service and practicing mathematics teachers and their students. In doing so, she proposes three reasons why teachers matter. She then gives some examples of "how" teachers can matter to their students in a more practical sense. (Contains 3 figures and 1 table.)