Lesson planning is integral to teachers' professional development in China. It includes their individual reflection and study as well as the collegial activities undertaken to prepare the lesson. In China, organizational structures for both individual teachers and a school's professional community embed lesson preparation in two activities: preparing a lesson plan and refining the plan through "open lessons." In this article, the authors explore not only how Chinese teachers develop lesson plans, but also how the organizational structure of Chinese teaching enables them to use lesson plans as a professional-development activity. Lesson planning allows teachers to explore multiple aspects of pedagogical content knowledge. In developing lesson plans, teachers have opportunities to think deeply about the subject matter, including the way the subject matter is represented in particular textbooks or in such aspects of the curriculum as standards and benchmarks. They also have time to develop pedagogical activities or methods that enable students to grasp the subject matter. Lesson planners can ponder what students know and how they may best understand the content.