Lesson study, the dominant form of professional development for teachers in Japan, has spread rapidly in the U.S. since 1999. In this article, the authors discuss the growth and success of lesson study at Highlands Elementary School in California's San Mateo-Foster City School District and identify conditions needed for scale-up. They also discuss the history and evolution of lesson study at Highlands school. Among other things, they discuss four changes in the larger education policy climate that they believe could help practice-based learning systems like that at Highlands to become widespread. These are: (1) cross-site learning about lesson study; (2) a diverse ecology of lesson study; (3) pathways linking lesson study to textbooks; and (4) provision for "inside-out" reform. (Contains 1 figure and 17 endnotes.)