The nature of mathematics teachers' knowledge specific to teaching mathematics [MTK] is of ongoing concern in mathematics education research. This article contributes to our under-standing of this knowledge with particular focus on reflective awareness. It discusses MTK based on ways it has been used in research. It highlights reflective awareness as a central aspect of MTK based on a study of elementary school mathematics teachers' learning and teaching associated with a self-directed professional development initiative to transform their teaching to an inquiry-based perspective. Research questions focused on how this initiative supported reflective awareness in the teachers' learning and teaching. Findings indicated that engaging in self-based and meaning-based questioning and creating pedagogical models were central to the teachers' learning and use of reflective awareness. Their knowledge of reflective awareness as a way of knowing was important to their development of an inquiry stance and knowledge of mathematics for teaching and mathematics pedagogy.