This document discusses the process of lesson study, an approach to teacher professional development originated in Japan. It involves teachers collaboratively planning, observing, and revising lessons with a focus on student thinking and mathematics learning. The goal is continuous improvement of teaching practice over time. The lesson study cycle includes setting goals, developing a lesson, teaching and observing the lesson, debriefing and revising, then repeating the process with reflection. Teachers engage as researchers investigating their chosen goals and student learning through this collaborative process of lesson planning and analysis.