The use of English as a language of instruction in higher education, commonly referred to as 'EMI' after 'English Medium Instruction', is becoming increasingly widespread throughout the world. In Sweden, for example, 64% of all education at master's level is now conducted in English. The overall knowledge of the consequences of EMI is inadequate, and particularly little is known about the effects of EMI on the learning of subject content. EMI typically means that neither teachers nor students have English as their first language, i.e. both teaching and learning take place in a foreign language. Consequently, teachers' English language skills are extremely important - a key factor for effective education is that students understand the content of the teaching. The overall aim of this project is to contribute to the long-term quality assurance of EMI by investigating the relationship between teachers' English language skills and the quality of teaching. The following two questions guide the work: (i) What are the English language skills of teachers teaching in EMI contexts? (ii) Is there a link between teachers' language skills and their attitudes to learning and the quality of learning? Data is collected in the form of language tests, teachers' description (self-assessment) of teaching and interviews with teachers. The study is a first step towards understanding what expectations universities can reasonably have of EMI teachers' English, and whether language proficiency is linked to the quality of learning.