Proficiency in English is a central consideration in English-medium instruction (EMI). One one hand, a satisfactory level of proficiency is needed in order to insure that students have good preconditions for success in their studies. On the other hand, increased skills in English are among the intended outcomes which lead universities to implement EMI (though not, of course, the only one). Perhaps surprisingly, relatively little is known about the relationship between EMI and English proficiency. This talk will marshall the available evidence in order to answer three questions: 1) what level of English proficiency do students around the world have when they enter the university-level EMI classroom? 2) what level of English proficiency gives good preconditions for success in EMI? and 3) to what extent does EMI raise proficiency levels?