Content and language integrated learning (CLIL) is a rapidly growing area of both research and practice in all parts of the world, especially in Europe and Asia. As a young discipline, CLIL has a good potential of distinguishing itself from monolingual L2 immersion education models by becoming more flexible and balanced about the role of L1 in CLIL lessons. Although recent years have witnessed increasing research on the potential role of L1 in foreign language teaching e.g. Littlewood, W., & Yu, B. 2009. First language and target language in the foreign language classroom. Language Teacher, 42, 1-14, monolingual immersion ideologies are still dominant in many contexts in the world (especially in Southeast Asia) because of a whole host of ideologies. The beliefs affecting medium of instruction policies and practice have their roots in the traditional tenets (e.g. the maximum input hypothesis) in the discipline of second language acquisition (SLA). Although these tenets are increasingly being countered by recent research in multilingualism see May, S. (Ed.). (2014). The multilingual turn: Implications for SLA, TESOL and bilingual education. New York: Routledge, for a critique of these tenets, SLA still has an influence on pedagogies in both immersion and CLIL programmes. In this paper, I shall first critically review these deep-rooted monolingual tenets. Then, I shall discuss how we can conceptualise the potential role of L1 in CLIL and by extension in content-based instruction (CBI), as both CLIL and immersion programmes are considered to be key approaches to CBI. I shall conclude with suggestions for future research in CLIL.