This article concerns the contribution that feedback makes to valid classroom assessment of the writing of young learners (YLs), defined here as approximately 9-13 years old. It shows that a scale of descriptors adapted from the "Common European Framework of Reference for Languages" (CEFR; Council of Europe, 2001) can play a central role in this assessment. The article presents a research project, AYLLIT (Assessment of Young Learner Literacy), which developed a CEFR-based instrument and guidelines for teachers, to provide their pupils with feedback that may allow them to see progress and lead to further learning. It sets out the procedures followed in the project and examines the extent to which its outcomes enable teachers to give feedback that contributes to valid classroom assessment.