Abstract: Written mathematical tests are traditionally associated with summative assessment, aimed at certifying students’ achievement. However, recent research highlights the potential of such tests to support formative assessment when their design and analysis focus on providing meaningful feedback for both teachers and learners. In this talk, I will discuss a set of design principles and interpretative tools for written mathematics tests that can enhance their formative function. Drawing on classroom-based studies, I will illustrate how data from students’ responses can be systematically analysed to produce feedback that supports teachers in three key functions of formative assessment: feed up (clarifying learning intentions and success criteria), feedback (identifying students’ current understanding), and feed forward (informing instructional decisions and next steps).