With the expanding role of English medium instruction (EMI) worldwide there has been an increasing use of English to teach content subjects at tertiary level. In implementing EMI a common assumption has been that the primary issue affecting successful EMI is the English proficiency level of the EMI instructor. This paper reviews this assumption by exploring three dimensions of EMI--the cognitive, the genre, and the academic task dimensions of teaching in a discipline--and the role that English plays in developing disciplinary literacy and competence. From a review of EMI teachers' accounts of EMI in Hong Kong as well as other EMI contexts, information is presented to demonstrate the ways in which EMI teachers adapt their teaching in order to prepare for and manage EMI. Goals are suggested for the professional development of EMI teachers.