Yuen Yi LO has been researching key issues related to the medium of instruction, especially using English as the medium of instruction in English-as-a-foreign-language contexts. Her early research focused on how using English as the medium of instruction affects classroom interaction processes and students’ learning outcomes, and she published several important papers on the topic in Research of Educational Review and International Journal of Bilingual Education and Bilingualism. Her paper “A meta-analysis of the effectiveness of English-medium education in Hong Kong” was awarded the Faculty Early Career Research Output Award and was one of the 10 most-read articles in Research of Educational Review in 2014. Based on the insights gained and her experience of working with teachers, Yuen Yi’s recent research focuses on teachers’ professional development in English-medium or Content and Language Integrated Learning contexts, which is a pressing, yet underexplored issue.