The purpose of this study was to examine elementary teachers perspectives on the impact that Lesson study participation had on their knowledge and teaching of mathematics. The study explored teachers perspectives of how Lesson study participation affected their mathematical content knowledge and pedagogical-content knowledge. Lesson study is a professional development practice with roots in Japanese schools. It has shown great promise in improving the quality of teaching mathematics in ways that are sustainable and generative in nature. Essentially, Lesson study is a collaborative method of goal setting, instructional planning, assessing its impact, and reflection and refinement of teaching. It is iterative in nature and requires that teachers participate for extended periods of time to reap the benefits. This study of elementary teachers from western Pennsylvania revealed that teachers perceived a positive impact on both their mathematical and pedagogical content knowledge. Specifically, these teachers reported changes in how prepared they felt to teach math, their understanding of core math concepts, how they understood the connections between math concepts, and how prepared they felt to effectively plan for instruction. When reporting their perceptions of the impact that Lesson study had on their pedagogical-content knowledge, teachers cited the greatest impact in their understanding of student misconceptions in math, how prepared they felt to teach concepts rather than skills, and how often they understood how children learn math. This study examined the challenges and/or enabling factors encountered while participating in Lesson study and the findings suggest that "time" was a major challenge--both scheduling time to meet and the amount of time needed to devote to Lesson study. The amount of administrative support the teachers received was both a major challenge and major factor enabling success. The most frequently cited major factor enabling success was the size and make-up of the Lesson study group. Overall, the findings in this study suggest that a majority of the teachers believed that Lesson study helped them become a better math teacher, and that it was an effective way to continue their professional development. A majority also reported that they plan to continue Lesson study participation. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]