In recent research, it has been argued that effective professional learning would continue over the long term and is best suited for a school community that promotes learning. This requires an examination of the teachers on a daily basis. In order to develop such a community, it becomes increasingly important for teachers to mutually observe and jointly reflect on practices at the classroom level. Educators in the United States have begun paying close attention to the Japanese tradition of lesson study. This paper aims to discuss the chief findings and issues that have hitherto been raised with respect to research on lesson study and to clarify current progress and limitations in the research in the United States and Japan.