In this article I build on the review of research by Black and Wiliam which provided extensive evidence of the role of assessment for learning for raising standards in pupils' attainment. The purpose of the present empirical study was to explore the extent to which science teachers use assessment for learning practices when they teach physics investigations to 11-18 year-old pupils. Nine secondary science teachers in four schools in the greater London area were observed when they taught theory and physics investigations during one school year. They were also interviewed and pupils' written work was collected. The research findings show that only a few of the participating teachers implemented some elements of assessment for learning practices. Implications for the design of questions in the classroom and written feedback in marking are discussed. It is illustrated how teachers' questions and written feedback in marking need to be based on the cognitive demands of each investigation so that they support learning. I argue that teachers' decisions on how to support pupils to improve their attainment in physics education have to be informed by teachers' subject and "pedagogical content" knowledge.