The model presented in this article aspires to represent a theoretically valid and coherent definition and description of writing, as a basis for teaching and assessing writing as a key competency in school. It represents a critique as well as an extension of previous alternatives in that it views writing as a culturally and individually intentional act of semiotic mediation. Its perspective is sociocultural and functional; focussing on the relation between writing acts and their purposes, and its dynamic construction captures flexible relations between these. The communicative aspects encapsulate semiotic tools which written mediation affords. A discussion of curricular and didactic potentials finishes the article, illustrating how the model may form a basis for a varied teaching of writing, as well as being a tool for learning and assessment within and across school subjects.