In Australia, and internationally, there has been an escalation of government investment (both rhetorical and financial) in improving the quality of teachers and teaching in order to enhance student outcomes and reduce equity gaps. Teaching standards, teacher institutes, government investment in the science of learning and measurement of quality, all signal growing urgency for finding ways to improve the quality of teaching. These initiatives rest, to a significant extent, on the capacity for and effectiveness of teacher development. In this paper we report outcomes of a project that sought to: 1. Test, on a rigorous scale, an approach to teacher professional development for its capacity to impact on teaching quality; 2. Explain the functioning of this approach to teacher development through analysis of impact on teacher identity, teaching culture, and teachers’ career trajectories. The approach, known as Quality Teaching Rounds, developed by Bowe and Gore, brings together the benefits of professional learning communities, instructional rounds, and the Quality Teaching framework (NSWDEC, 2003). The underlying premise of the Quality Teaching Rounds approach is that attention must be given to both the form and substance of professional development. By adding a pedagogical framework to collaborative ways of working, Quality Teaching Rounds provide teachers with a common language and set of conceptual standards with which to engage in rigorous diagnostic conversations focused on their individual and collective practice. A cluster randomised controlled trial was conducted, involving two ‘intervention’ groups (a ‘set’ intervention and a ‘choice’ intervention) and a waitlist ‘control’ group. Two full lessons per teacher for 192 teachers in 24 schools (8 in each group) were observed by the research team (blinded to group allocation) at three time points: before Quality Teaching Rounds commenced, 6 months later when the two intervention groups had finished, and again 6 months after that (total ~1150 lessons) in order to consider the sustainability of any effects. Surveys, interviews and case studies enabled analysis of impact on teachers’ identities, teaching culture and teachers’ career commitments. Pending 12 month data, significant effects were found at 6 months for the primary outcome, Quality Teaching, (‘set’ p<0.001, d=0.38, ‘choice’ p=0.047, d=0.20) demonstrating the efficacy of this approach to teacher development. The approach not only impacted on the quality of teaching but was embraced by teachers who reported positive effects on confidence in their capacities, their professional learning, and their productive relationships with colleagues.