English Medium Instruction (EMI) in universities continues to grow as an educational phenomenon and increasingly is attracting the attention of researchers, particularly researchers who are applied linguists. Starting with a definition of EMI and with the challenges that any working definition poses in the many contexts in which EMI is said to be operating, this paper explores the extent to which some of the theories prevalent in the field of second language acquisition can be transferred to the kinds of content classrooms which are labelled EMI. Examples of language used in content classrooms are provided and analysed with two specific foci: what and how vocabulary is used by teachers; the use of the learners' first language by the teacher. Some recommendations for taking a more nuanced perspective towards EMI research by applied linguists are suggested.