This research focuses on the use of teaching resources that support an ambitious student-centred approach to learning mathematics. A multiple case study involving three teachers of post-16 mathematics is presented to develop new insights into the process of teacher change. Data sources include classroom observations in addition to teacher and student interviews. Using the concept of Zone of Enactment (ZoE) and teacher self-efficacy as a theoretical framework this research reveals the importance of teachers being aware of, and able to reflect on, the ZoE and their self-efficacy.