There is a continuing emphasis in the United States to improve student's mathematical abilities and one approach is to better prepare teachers. This study investigated the effects of using lesson study with preservice secondary mathematics teachers to improve their proficiency at planning and implementing instruction. The participants were students (preservice teachers) in an undergraduate teacher preparation program at a private university who were enrolled in a mathematics methods course for secondary math teachers. This project used lesson study to engage preservice teachers in collaboratively creating lessons, field testing them, using feedback to revise the lessons, and re-teaching the revised lesson. The preservice teachers worked through multiple cycles of the process in their secondary math methods class receiving feedback from their peers and instructor prior to teaching the lessons in their field experience (practicum). A mixed methods approach was implemented to investigate the preservice teacher's abilities to plan and implement instruction as well as their efficacy for teaching. Data were collected from surveys, video analysis, student reflections, and semi-structured interviews. The findings from this study indicate that lesson study for preservice teachers was an effective means of teacher education. Lesson study positively impacted the preservice teachers' ability to plan and teach mathematical lessons more effectively. The preservice teachers successfully transitioned from teaching in the methods classroom to their field experience classroom during this innovation. Further, the efficacy of the preservice teachers to teach secondary mathematics increased based on this innovation. Further action research cycles of lesson study with preservice teachers are recommended. [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.]