Peer Assessment (or Peer Review) is a popular form of reciprocal assessment where students produce feedback, or grades, for each others work. Peer Assessment activities can be extremely varied with participants taking different roles at different stages of the process and materials passing between roles in sophisticated patterns. This variety makes designing Peer Assessment systems very challenging. In this paper we present a number of Peer Assessment case studies and show how a simple review cycle can be used as a building block to achieve the more complex cases. We then propose a Canonical Use Case for Peer Assessment, in which a Review Plan is used to describe how review cycles can be combined to achieve the required complexity.