Within an undergraduate programme, a four-year action research project implemented, evaluated and refined a regime of peer assessment focused on generating high-quality peer feedback. Changes in structure and process transformed a system that had initially been characterised by a reluctance to criticise fellow students into one that produced immediate, reflective and useful peer feedback. The two key factors in this transformation appear to be: making peer assessment openly and exclusively formative; and vesting ownership of all data generated by the process in the student being assessed.