Abstract Some authors have noted a lack of conceptual clarity in the field of L1 grammar instruction for writing. Here ‘metalinguistic activity’ is proposed as a concept that can contribute conceptual clarity by relating metalinguistic activity both to the reflexivity that languages afford (i.e., using language to refer to language itself) and to a sociocultural approach to languages (i.e., languages consist of semiotic tools that underpin our psychological development). I discuss how this notion is approached by Grup de Recerca sobre Ensenyament i Aprenentatge de Llengües (GREAL), the Research Group on Teaching and Learning Languages at the Autonomous University of Barcelona (Spain), and describe their seminal study of secondary students' metalinguistic activity in the context of writing. Results show that metalinguistic activity emerges at different levels (procedural, with common language, and with metalanguage). While it may not lead to students automatically writing better texts, metalinguistic activity does help them to engage in sustained discussion about text choices in the context of text production, something considered of utmost importance in educating good writers. In conclusion, I indicate the theoretical relevance of this concept and suggest the need for more research on how to implement it effectively in the classroom.