The idea that schools can impact positively on student outcomes is a crucial driver in the rise of interest in school improvement research and practice. This review focuses on assessment for learning. Assessment for learning (AfL)--where the first priority is to promote learning--is a key means of initiating improvement. This review proposes that in order to encourage AfL, subject departments: (1) have an atmosphere in which teachers are expected to watch others in action--to actively support peer observation; (2) recognise and value current skills and help teachers to identify their current formative practice; (3) have meetings where teachers discuss learning; and (4) give teachers time to plan well by encouraging them to mark less, but mark better. The review concludes by identifying two sets of guidelines for policymaking and practice.