Understanding and using assessment data is at the heart of assessment for learning, as indicated by the Assessment Reform Group’s definition of AfL: ‘The process of seeking and interpreting evidence for use by learners and their teachers to decide where the learners are in their learning, where they need to go next, and how best to get there’ (ARG, 2002) (italics added). In order to interpret assessment evidence appropriately we need to understand how it was generated, its limitations and the confidence we may have in it. Then we can decide how to use the information to aid pupils’ learning, by knowing more clearly what their precise needs are, what will engage them in their learning and how their learning can be supported within and beyond the classroom. This process has now also become known as ‘personalising learning and teaching’, defined by the ‘Teaching and learning in 2020 review group’ as ‘. . . taking a highly structured and responsive approach to each child’s and young person’s learning’ (DfES, 2006a: 6).