This paper reports the findings of a study of the medium of instruction in the engineering stream at tertiary level in Hong Kong. The article begins with overview of developments in language in Hong Kong education in the post-war period, and then goes on to describe and discuss the small body of research into language use that has been carried out in the territory's universities in the 1990s. The focus of the paper is a study of the use of English, Chinese and Cantonese as media of instruction in an engineering degree course (offered in full-time and part-time modes) at a representative English-medium technical university in Hong Kong. It was found that English is the main language of written assignments, textbooks and course materials. The findings relating to oral communication are much less clear. English tends to be used by students and lecturers in more formal situations such as lectures, presentations and seminars, while in less formal situations such as tutorials and discussions, Cantonese seems to have an important role. The findings also indicate that both students and lecturers on the part-time course make greater use of English than their counterparts on the full-time course.