esson Study is a model of teacher professional development that has seen a worldwide increase in popularity in recent years. Teachers collaboratively review their approaches through planning and reflecting on cycles of 'research lessons'. Dialogue therefore becomes a core mechanism in the development of their learning and practice. This chapter aims to provide an overview of the nature and potential of Lesson Study professional dialogues. It draws on data from one large-scale project with Mathematics teachers from primary and secondary schools in London. Through a qualitative analysis of case study examples, the chapter aims to demonstrate features of these dialogues that are productive in relation to subsequent practice development. Implications for teacher professional development are discussed.