An enduring issue in immersion education focuses on the appropriate use of the L1 in the one-way or two-way immersion classroom. This article discusses several key constructs (mediation, languaging, the cognition/emotion relationship, zone of proximal development) that are central to a Vygotskian sociocultural theory of mind perspective on second language learning and teaching. Each discussion of a theoretical construct is followed by a review of one or more key research studies from one-way or two-way immersion contexts whose findings are highlighted or re-interpreted in light of Vygotsky’s insights. The theory and research yield three important guiding principles with the goal of helping educators to make decisions about their language use choices in the immersion classroom.