This chapter proposes a theoretical model of the impact of lesson study. Outcomes addressed include teacher outcomes (e.g., knowledge and beliefs), professional learning norms and routines, instructional routines and tools, and student learning outcomes. Four theoretical perspectives are used to examine lesson study impact: knowledge integration environment, self-determination theory, self-efficacy theory, and pedagogies of practice. The chapter also examines all four phases of the lesson study cycle -- study, plan, teach, and reflect -- and for each phase identifies major goals, challenges, strategies to overcome challenges, and relevant theoretical perspectives. In addition, reflection questions for each phase are proposed, which are designed to support educators and researchers to reflect on the effectiveness of their work during each phase. The chapter is based on 20 years of observations of lesson study and is intended to spark further conversation about the process and impact of lesson study. [This chapter was published in R. Huang et al. (Eds.), "Theory and Practice of Lesson Study in Mathematics," Advances in Mathematics Education (p13-37). Cham, Switzerland: Springer Nature.]