This article develops a theoretical understanding of the processes involved in the co-construction of a written text by a teacher and student from a Vygotskian perspective. Drawing on cultural-historical and sociocultural theories of writing and Vygotsky's concept of the Zone of Proximal Development (ZPD), this case study of a student and teacher interaction in a UK secondary school examines the social mediation of collaborative activity in the negotiation of meaning. While expressivist process theories of writing focus on the development of the authentic voice of the writer, this article contends that the development of a student's writing abilities requires active intervention by a teacher within a constructed zone of development. Writing is viewed as a situated activity system that involves a dialectical tension between thought and the act of composition. Finally, the article will argue that the recursive and complex nature of writing development is an integral tool in the learner's own agency in creating a social environment for development. (Contains 4 figures and 1 note.)