This article presents a theoretical model of lesson study's impact on instruction, through intervening impact on teachers' knowledge, beliefs and dispositions, teachers' learning community, and curriculum. It also describes four different types of lesson study in Japan, pointing out their synergies in producing a system where local teachers "demand" knowledge for their lesson study work and lesson study provides a collaborative, practice-based venue to try out recent innovations in curriculum and instruction. Description of lesson study in Japan provides background for considering the articles of this issue, which highlight four strategies to develop lesson study outside Japan: (1) incorporation of high-quality tasks and materials; (2) attention to processes that illuminate student thinking; (3) attention to system features; and (4) models for scale-up.