Under the label of Content and Language Integrated Learning (CLIL) the teaching of curriculum subjects through the medium of a foreign language has become a widely accepted feature in mainstream education systems in Europe and other parts of the world. After contextualizing its subject matter in CLIL research as a whole, this article focuses on research into classroom discourse. In order to unravel the complexities involved, three different takes on CLIL classroom discourse are discussed as an evidence-base for (a) language learning, (b) language use and social-interactional aspects of L2-interaction, and (c) processes of knowledge construction in and through a second or foreign language. The article concludes with an outline of requirements for further research in the area.