Using video recordings of lessons and interviews with teachers, this article explores the way in which teachers enact Assessment for Learning (AfL) practices in their classrooms. Starting with the hypothesis that AfL is built on an underlying pedagogic principle that foregrounds the promotion of pupil autonomy, we analyse the ways in which teachers instantiate this principle in practice. A distinction is drawn between lessons that embody the 'spirit' of AfL and those that conform only to the "letter". The nature and sequence of tasks and especially 'high organization based on ideas' appears crucial to the former. This adds a dimension to more familiar formulations of AFL practices. We also ask whether the teachers beliefs about learning contribute to the different ways in which they interpret the procedures of AfL. Interviews with teachers indicated that those whose lessons captured the spirit of AfL were more likely to take responsibility for success and failure in the promotion of pupil autonomy. Thus they had a sense of their own agency and sought to use it to overcome barriers to learning.