Lesson Study is a professional development model that supports teachers working together to analyze and improve classroom lessons. By working over the course of a year to write, study, teach, revise, and re-teach between one and three lessons, teacher energies are focused on improving mathematics instruction. While the lesson is taught, others observe and collect data about students’ responses—a practice that leads to lesson revision. This long-term process allows a team of teachers to think carefully about orchestrating the complexities of instruction and studying its effects on student understanding.