Lesson study cultivates teachers' capacity for formative assessment by placing student thinking front and center throughout. Lesson study is a form of professional development in which a team of teachers determines a mathematical focus, collaboratively studies student thinking about the topic, designs a lesson about this content, implements the lesson while collecting detailed evidence of student learning, and reflects on the impact of the lesson on student learning and behavior. The lesson study process encourages careful observation and analysis of student thinking, with the goal of designing and implementing effective teacher responses to student actions. There are four phases of lesson study: (1) Study; (2) Plan; (3) Teach/Observe; and (4) Reflect/Modify. In this article the authors illustrate the phases of lesson study by analyzing a lesson about piano practice.