The present paper offers a definition of peer assessment and then reviews the major syntheses on its effectiveness. However, the main part of this paper is preoccupied with how to do PA successfully. A typology of 44 elements explains the differences between the many types of peer assessment. Then a theoretical model outlines some of the processes which may occur during PA. Initially, only a few of these will be used, but as those engaged in PA become more experienced, an increasing number of elements will feature. However, these may not appear in the linear order set out here, and indeed may be recursive. The implications for the design and organisation of PA are outlined, as well as the implications for future research.