As digital technologies develop and change, so do the ways these tools are integrated into classrooms. In particular, as mobile digital technologies become ubiquitous there is a need to investigate how teachers engage with these tools--both personally and professionally. Research has consistently shown that teachers' underlying beliefs and attitudes (particularly their self-efficacy beliefs) are key elements that influence use and integration of digital technologies in the classroom. In this paper, changes to and factors influencing teachers' mobile digital technology self-efficacy beliefs, and their subsequent classroom use of devices, are examined in the context of a one-to-one iPad mini device programme in an international school. Results indicate that all of the teacher participants reported an increase in the use of the iPad mini in the classroom, partly as a result of students' development of collective efficacy. As well as this collective efficacy, which supported the increased use of devices, other factors supported the development of teacher self-efficacy. These included modelling and coaching from colleagues, but mastery (or actual experience) was the foremost contributor to the development of teachers' mobile technology self-efficacy. This study revealed that allowing teachers time to experience mastery in relation to mobile technology use, and having access to expertise (both colleagues and students), were key elements in building self-efficacy for teachers over time. Perceptions of device value and device affordances were also identified as factors that fostered the development of self-efficacy and subsequent mobile device implementation and use.