Peer assessment is recognized as a useful learning activity, not merely as a means by which assignments can be marked. In a design subject such as Human- Computer Interaction, peer-assessment offers a unique opportunity for students to be exposed to a wide range of different designs in an environment that ensures that they reflect on these designs. However, it is important that the marking criteria are well specified and unambiguous.This paper reports on the use of peer-assessment as an appropriate learning activity for revealing the wide range of design issues and dimensions in interface design, where formal marking is based on clear, functional criteria. Subjective consideration of the interfaces is encouraged through a ranking system and the subsequent elicitation of interface design principles. The successful implementation and acceptance of this scheme demonstrates its benefits both as a learning activity and as an opportunity for student reflection.