This new edition of Unjust Enrichment by the editor of the Clarendon Law Series, is a fully updated, clear and concise account of the law of unjust enrichment. It attempts to move away from the use of obscure terminology inherited from the past. This text is the first book to insist on the switch from restitution to unjust enrichment, from response to event. It organises modern law around five simple questions: Was the defendant enriched? If so, was it at the claimant's expense? If so, was it unjust? The fourth question is then what kind of right the claimant has, and the fifth is whether the defendant has any defences. This second edition was revised and updated by Peter Birks before his death from cancer on 6 July 2004 at the age of 62. It represents the final thinking of the world's leading authority on the subject.