Given the distinctive characteristics of Japanese mathematics lessons found by the international studies and the subsequent focus of the attention to Lesson Study originated in Japan in education community, the author discusses the relationships between the scientific studies and endeavour for the improvement of teaching and learning from an insider’s perspective. Reflecting on how mathematics educators have been struggling with studying the complex phenomena called "lesson" in the Japanese context, it is argued that orchestrating the scientific goal of building theories and the goal of improving teaching and learning of mathematics is the key to a synergy between research and the more practical knowledge of the craft of teaching.