Most studies of EFL classroom discourse focus on interaction. Teacher monologue, however, has received scant attention, despite its important role in providing students with input. This study investigates semantic development in teacher monologue. The approach is transdisciplinary, jointly employing Systemic Functional Linguistics (SFL)and Legitimation Code Theory (LCT). Semantic development is first analysed in terms of semantic patterns revealed by grammatical choices within the framework of SFL. Next, the concept of semantic gravity in LCT is referred to for the display of how the semantic patterns unfold over time. Finally, based on relevant studies, the article discusses possible influences of semantic development in teacher monologue on students’ comprehension of texts and construction of knowledge.