Lesson study is a model of effective professional development that originated in Japan and has become popular among math teachers worldwide in recent decades. Idaho math teachers have engaged in lesson study as a method of professional development for the past six years. Research is needed to determine the impact of lesson study on the professional growth of math teachers in Idaho. This study used a convergent mixed methods design to investigate the impact of lesson study on math teacher professional growth using the Danielson Framework for Teaching (FFT). The researcher used the Danielson FFT classroom observation instrument to identify the professional growth of lesson study participants. The researcher also used semi-structured teacher focus group interviews with teachers and one-on-one interviews with administrators to gain their perspectives regarding the impact of lesson study on math teacher professional growth. The data indicated lesson study with teachers had shifted math instruction from a traditional lecture-based approach to a more student-centered, discovery learning approach. Specifically, lesson study participants had created safe learning environments by normalizing error as part of the learning process. As a result, students were taking intellectual risks during class discussions and persevering in solving problems. Lesson study showed a positive impact on the professional growth of math teachers in the areas of question and discussion techniques, engaging students in learning, and using assessment during instruction. Results suggest lesson study can be used as an effective model of professional development for math teachers. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]