Developing sustainable professional development which facilitates teachers of mathematics to develop effective mathematics pedagogy has been a key aim in recent years. This paper examines how lesson study can be used with networks of teachers as a vehicle to promote and sustain professional development. Drawing on findings from a year-long study involving four schools, the paper highlights how through the process of collaboratively planning a lesson, observing and discussing it, teachers were facilitated to adopt approaches to mathematics teaching that are aligned with the factors identified as effective mathematics pedagogy. It also illustrates how lesson study can support teachers to engage in a collaborative network, develop their professional knowledge, and reflect on their teaching practice. (Contains 1 table.)