Classroom discourse has a special place in the history and study of second language acquisition. From the early and seminal work of Evelyn Hatch in the 1970s to current and cutting-edge investigations of multimodal interactions, the discourse of classrooms is critical to the advancement of theories and hypotheses of second language acquisition. Furthermore, classroom discourse is central to how scholars from traditions ranging from psycholinguistics to conversation analysis understand the ways in which second language learning develops and unfolds. In this chapter, an overview of such work is provided, focusing on investigations that look at interaction from multiple theoretical and methodological perspectives. The long and rich history of using classrooms to investigate second language acquisition is discussed. Special attention is paid to how classrooms present unique contextual affordances and constraints for second language acquisition. This is done by attending to the many languages and regions that have been investigated in the literature.