Formative assessment has attracted increasing attention from both practitioners and scholars over the last decade. This paper draws on the authors' empirical research conducted over eleven years in educational situations ranging from infant schools to postgraduate education to propose a theorisation of formative assessment. Formative assessment is seen as taking place when teachers and learners seek to respond to student work, making judgements about what is good learning with a view to improving that learning. However, the theorisation emphasises formative assessment as being a discursive social practice, involving dialectical, sometimes conflictual, processes. These bring into play issues of power in which learners' and teachers' identities are implicated and what counts as legitimate knowledge is framed by institutional discourses and summative assessment demands. The paper argues that, rather than only paying attention to the content of learning, an ambition for formative assessment might be to deconstruct these contextual issues, allowing a critical consideration of learning as a wider process of becoming. The article suggests a model that might be useful to teachers and learners in achieving this. (Contains 6 notes, 3 figures, and 2 tables.)