Most readers of the Kappan will know that lesson study is a Japanese approach to teacher professional development that has recently become popular in the U.S. Indeed, lesson study activity has been spreading rapidly for several years and continues to proliferate today. However, to date, most of the discussion and research about U.S.-based lesson study has focused on how individual groups of teachers can implement this practice or become engaged in it more deeply and fruitfully. Professional development fads come and go, with little lasting impact. But as lesson study takes hold in the U.S., it increasingly reveals its potential to advance the entire teaching profession. In this article, the authors assess the current state of U.S. lesson study and detail how its practitioners can capitalize on and extend its influence.