When teachers ask questions or devise activities that encourage pupils to talk, they set up opportunities for assessment. To act on these data, teachers need to convert judgements into decisions about next steps and guidance for pupils on how best to move their learning forward. This is a complex process that requires skill and action, both for the teacher and the pupils. This article considers how classroom assessment functions for science in classrooms in England. It explores some of the opportunities and dilemmas that might occur in the current and developing school assessment frameworks in an attempt to raise issues that teachers might need to consider as they implement new assessment systems in their classrooms.