In this article, Merrilyn Goos looks at beliefs about the nature of mathematics and perceptions of mathematics teachers in examining "why teachers 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. Goos illustrates some of these trends by drawing on her experiences working with pre-service and practising mathematics teachers and their students, and in doing so proposes three reasons why teachers matter. She then gives some examples of how teachers can matter to their students in a more practical sense. [This article was first published in the "Australian Mathematics Teacher," in 2006 (EJ765835).]