Without assuming prior legal knowledge, books in the Directions series introduce and guide readers through key points of law and legal debate. Questions, diagrams and exercises help readers to engage fully with each subject and check their understanding as they progress. This chapter discusses the various ways of classifying mistakes including communication mistakes, mistakes of fact, common mistake and unilateral mistake. It then looks at the case law on mistaken identity and the distinctions between face-to-face and correspondence contracts. Finally, it looks at the restrictive rules on common mistake, including the difference between fundamental mistake and mistake as to quality, and their relationship with the doctrine of frustration and the demise of the category of equitable mistake.