In Before It's Too Late: A Report to the Nation from the National Commission on Mathematics and Science Teaching for the 21st Century (2000) in the US, the authors quote from James Stigler's conclusions from various videotape research studies of mathematics teaching: "The key to long-term improvement �in teaching� is to figure out how to generate, accumulate, and share professional knowledge." Japanese Lesson Study has proved to be one successful means. This book supports the growing movement of lesson study to improve the quality of mathematics education from the original viewpoints of Japanese educators who have been engaging in lesson study in mathematics for professional development and curriculum implementation. This book also illustrates several projects related to lesson study in other countries.