This paper concerns the conceptual and pedagogical issues that revolve around target language (TL) only instruction and teacher code-switching in the context of TL classrooms. To this end, I first examine four intertwined ideas (that is, monolingualism, naturalism, native-speakerism, and absolutism) that run through the monolingual approach to TL teaching, with a discussion of why these assumptions cannot hold in light of what is actually going on in the world today--bilingualism. The paper then reviews contemporary classroom code-switching research within a sociolinguistic framework, arguing that teacher code-switching should be permitted as a legitimate pedagogical practice, as it is not only an example of natural bilingual behaviour, but it also has great potential in terms of contributing to the development of TL learners' bilingual competence. It is further argued that newly emerging studies on the effects of teacher code-switching on TL acquisition provide an important pedagogical rationale for its inclusion in TL teaching practices. Based on these discussions and research evidence, I propose a preliminary model of the effectiveness of teacher code-switching, which is designed to contribute to advances in both theory and teaching practices. (Contains 1 figure and 1 footnote.)