Different communities, speaking different languages, employ different naming systems to describe the events, actions, and interactions of the mathematics classroom. The International Classroom Lexicon Project documented the professional vocabulary available to middle-school mathematics teachers in Australia, Chile, China, the Czech Republic, Finland, France, Germany, Japan, Korea, and the United States. National teams of researchers and experienced teachers used a common set of classroom videos to stimulate recognition of familiar terms describing aspects of the mathematics classroom. This book details the existing professional vocabulary in each international community by which mathematics teachers conceptualise their practice, and explores the characteristics, structures, and distinctive features of each national lexicon. This book has the potential to enrich the professional vocabulary of mathematics teachers around the world by providing access to sophisticated classroom practices named by teachers in different countries.