Abstract-Why do bilingual language teachers and students switch between the two languages in their language classrooms? On the evidence of current research findings in relation to English-Vietnamese codeswitching in the educational contexts of Vietnam, this article identifies that classroom code-switching between the second language and the first language has its own pedagogic functions and it can be a valuable language classroom resource to both teachers and learners. In Vietnam, the implementation of the monolingual approach of teaching English-through-English-only faces many challenges such as inadequate classroom resources, students' low levels of English competence, motivation and autonomy, teachers' limited English abilities, and inappropriate teaching methods. Many Vietnamese teachers of English support code-switching in the classroom and they teach English through the bilingual approach. English-Vietnamese code-switching is reported not to be a restriction on the acquisition of English; rather, it can facilitate the teaching and learning of general English in Vietnam. This practice of code-switching is not just due to a lack of sufficient proficiency to maintain a conversation in English; rather, it serves a number of pedagogic functions such as explaining new words and grammatical rules, giving feedback, checking comprehension, making comparison between English and Vietnamese, establishing good rapport between teachers and students, creating a friendly classroom atmosphere and supporting group dynamics.