According to Vygotsky, we use tools both to shape and make sense of the world and to exercise control over others and over ourselves. Importantly, the very use of those tools to mediate and regulate our actions has developmental repercussions in our cognitive capabilities. From a sociocultural theory perspective on second language learning, these ideas are fundamental to our understanding of theoretical, pedagogical, and methodological issues. This paper reports on a pilot study framed within the principles of Concept-Based Instruction (CBI). The study involved six L2 advanced English learners doing postgraduate courses at a British university. Drawing on tests and protocols obtained by transcribing individual (think-aloud) and dyadic (pair-work) activity, our findings revealed that the CBI process helped most of our participants gain a deeper understanding of the concept of tense-aspect marking in English. Insights into the role of verbalisation as a regulatory tool were also gained through microgenetic analysis of the data. On the basis of our findings, some implications of CBI are also discussed in relation to grammar teaching and the value of metalinguistic knowledge in the L2 learning and teaching context. (Contains 3 figures, 4 tables, and 4 notes.)