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. In the same way, widespread feelings of unease amongst students regarding the concept of peer assessment were replaced with an enthusiastic engagement with the peer-feedback process. 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. In the refined system, peer assessment became part of a self assessment process rather than being either a distinct process in its own right or a part of the information-gathering attendant on teacher assessment. A particularly valuable finding of this study is that when peer assessment was focused on deriving quality feedback, students' mistrust of the process decreased steeply.