Peer assessment is very various in its implementation. Six studies of peer assessment are reviewed, four of them in higher education. A literature review is followed by five empirical studies. Strengths and weaknesses of each study are considered and issues are raised. Variables in peer assessment needing further exploration are extricated – in sample definition, specification of type of peer assessment, organisational nature of the intervention, and the measures which are used to evaluate change. It is suggested that further experimental and quasi-experimental studies are necessary to contrast one or two of these variables at a time.