Classroom observation has become a tool for analysing and improving English Language Teaching (ELT). This book represents the state of the art in language education and classroom interaction research from a data-driven empirical perspective. The micro-analytic, multimodal, and videographic approaches represented here understand classrooms as sites of complex, naturally occurring interaction. The volume demonstrates that the investigation of this communicative setting is the basis for insights into the inner workings of classrooms and the development of strategies for teacher education. The introductory article complements the volume by giving a comprehensive overview of the theories and methods that have come to bear in classroom observation.