In The Teaching Gap (1999), James W. Stigler and James Hiebert describe differences between mathematics instruction in Japan and the United States. They attribute some of these differences to a commonly used method of professional development in Japan called lesson study: “The lesson-study process has an unrelenting focus on student learning. All efforts to improve lessons are evaluated with respect to clearly specified learning goals, and revisions are always justified with respect to student thinking and learning” (p. 121). Over the past decade, many groups of educators in North America have implemented various forms of lesson study.