It is widely acknowledged that what teachers believe influences their teaching, yet the focus of much professional learning remains on influencing the specific practices and tools that teachers employ in their classrooms. In this article, it is argued that a greater and more explicit focus on teachers' beliefs would be beneficial. To this end, an overview of aspects of the understandings of the nature of beliefs is presented followed by findings from a recent study that examined mathematics teachers' beliefs and their impact on classroom practice. Finally, implications for mathematics teachers and those involved in designing and implementing professional learning for both teachers and pre-service teachers are suggested.