Ever since the beginning of the twentieth century, language, widely recognized as the main source of all things human, has been gradually becoming the principal object of study in all human sciences. Mathematics education is no exception. Since its inception, this research has already generated two language-related lines of study, one of them devoted to the uses of different, possibly multiple languages in the mathematics classroom, and the other, to forms of communication, or discourses, typical of learning-teaching processes. It is this first strand of study, to be called here language-in-mathematics-education (LiME) research, that constitutes the topic of this chapter. After some explanations on why and how language came to feature so prominently in researchers’ stories of mathematics teaching and learning, a comprehensive agenda for mathematics education research on language is presented and discussed.