This qualitative case study examines the professional development practice of Lesson Study as a vehicle for teacher learning. Through observing the participation and analyzing interviews of a group of United States middle school mathematics educators as they engaged in Lesson Study, this study examines teachers' learning with respect to (a) mathematical knowledge, (b) beliefs about teaching and learning, (c) study of teaching resources, and (d) participation in the social practice. Despite continued efforts by educators, some researchers assert that teacher professional development in the United States has not improved classroom instruction. For this reason, researchers have looked beyond the U.S. to international models to advance teacher learning. The "Trends in International Math and Science Study" (TIMSS), a prominent international comparison study, has brought into awareness countries that have models for improving teachers' practice. The Japanese model of Lesson Study (Jugyokenkyu) was identified as a highly developed form of professional learning that facilitates the transmission of expert teaching knowledge among colleagues. Researchers proposed that this Japanese professional development approach had the potential to improve teaching and learning in U. S. classrooms. Other literature also points to the Japanese model of Lesson Study as an exemplary standard of how to share knowledge of best teaching practices among teachers by providing teachers the opportunity to learn from each other. Limited research exists examining teacher learning in cases of Lesson Study as used in U.S. school settings. This case study adds to the literature by offering an understanding of how Lesson Study influenced the learning of a group of United States middle school mathematics educators. Further, this study provides insights into how the practice of Lesson Study impacted teachers' learning in order to offer evidence on ways Lesson Study could improve teaching and learning in other U.S. schools. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]