The purpose of this qualitative case study was to determine the effect of lesson study on teachers' mathematical understanding. This study reviewed a three year program called SEISMIC which implemented lesson study within a school district. The goal was to increase teacher and student understanding of mathematics' Teacher understanding of mathematical concepts did increase in part do to teacher collaboration that was vertical and horizontal. Teachers also began to realize the need to allow students time to develop their own understanding of the concepts being taught. This greater understanding translated to positive growth on the state standardized test in grades five through eight. Recent approaches such as Backwards Design, Differentiated Instruction, Authentic Learning, and the Japanese lesson study, are just a few reform ideas being implemented throughout the United States to increase student understanding. Many of these recent approaches to instruction are as a result of the TIMSS reports and the initiatives of NCLB. Many curriculum reform approaches fail however within a short time due to current cultural educational practices that exists in the United States such as teacher isolation. Educational reformers point out the significance of the TIMSS report and the Teaching Gap by Stigler and Hiebert (1999) as a pivotal wake-up call to U.S. education. Over a decade has past and reformers and proponents of change are still seeking ways to improve educational practices. Lesson study impacts multiple areas of education such as curriculum, professional development, teacher collaboration, and lesson research. Many other reform measures do not address these areas and therefore fail. If education reform is to succeed multiple areas of reform must be addressed. [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.]