Previous studies of in-service teachers indicate strong links between teacher self-efficacy and factors such as instructional quality and pupils' achievement. Yet, much of this research approaches self-efficacy from the perspective of teaching, and not of subject knowledge. Furthermore, the majority of such studies employ quantitative measures of self-efficacy. Drawing on semi-structured interviews with 22 experienced elementary teachers, this paper takes a different approach. The interviews, broadly focused on teachers' mathematics-related beliefs, brought to the surface four themes around which teachers construct their mathematics teacher self-efficacy. These concern participants' perspectives on their mathematics-related past experiences, mathematical competence, ability to realise their didactical visions and resilience in the face of challenging mathematical situations. These themes, which are discussed in relation to existing literature, not only confirm the complexity of self-efficacy but also highlight the need for greater attention to its conceptualisation and measurement.