"Lesson study" is a professional learning approach in which teachers work together to: formulate goals for student learning and long-term development; collaboratively plan a "research lesson" designed to bring to life these goals; conduct the lesson in a classroom, with one team member teaching and others gathering evidence on student learning and development; and discuss the evidence gathered during the lesson, using it to improve the lesson, the unit, and instruction more generally. A lesson study case from a Japanese elementary school is presented. Three types of knowledge developed by the teachers in the case are discussed: knowledge of the subject matter and its teaching-learning; development of the interpersonal relationships among teachers; and development of teachers' personal qualities and dispositions. The case suggests that development of a professional knowledge base for teaching may be an even more complicated endeavor than generally recognized, since it entails development of the knowers as well as the knowledge, and development of communities of practice, as well as individual teachers. (Contains 1 table, 2 figures and 4 notes.)