The New Zealand Numeracy Development Project is an example of a professional learning and development initiative that has been progressively scaled up across a system to improve teacher knowledge and practice and student outcomes in mathematics. This paper examines two elements of the project's design that have been pivotal in enabling teachers to adopt ambitious pedagogical practices in mathematics: classroom-focused opportunities to learn and access to external expertise. Four aspects of facilitator practice that impact on teachers' practice are identified: a focus on students' mathematical thinking; the use of pedagogical tools to structure teaching tasks; modelling and the provision of commentary; and observation and the provision of feedback. The question of sufficiency of professional learning and development opportunities is raised, particularly in enabling all teachers to respond to the challenge of providing equitable opportunities to learn for a diverse student population. The challenge of how systems can build the capability of facilitators so that they can engage teachers in substantive new learning and practice is highlighted as an important area for further investigation.