Two major concerns in mathematics teacher education are the role of subject matter knowledge and the development of self-efficacy in pre-service teachers. This article brings these issues together in an exploration of the interaction between pre-service teachers' perceptions of their subject matter knowledge and their accounts of university and placement experiences as potential sources of self-efficacy. Reporting on a group of ten pre-service teachers in Norway, we explore variations in the ways in which they perceived the role of subject knowledge in relation to experience, particularly "mastery experiences", over a period of nearly two years. We suggest that recognition of the role of "understanding why" in mathematics is crucial in the experience of mastery, and that there is a need to focus more on the role of subject matter knowledge in all sources of self-efficacy in teaching mathematics.