This volume explores a highly topical issue in second and foreign language education: the spreading practice in mainstream education to teach content subjects through a foreign language. CLIL has been enthusiastically embraced as a language enrichment measure in many contexts and finally research can offer principled insights into its dynamics and potentials. The editors’ introductory and concluding chapters offer a synthesis of current CLIL research as well as a critical discussion of unresolved issues relating both to theoretical concerns and research practice. The individual contributions by authors from a range of European contexts report on current empirical research in this dynamic field. The focus of these chapters ranges from theoretical to empirical, from learning outcomes to classroom talk, examining both the written and spoken mode across secondary and tertiary educational contexts.