This is a review article of Peter Birks, Unjust Enrichment (OUP, 2003) , criticising the argument for the introduction into English law of a doctrinal category of unjust enrichment, and discussing the role of justificatory and formal categories in the law and the nature of Birks's classification. It discusses restitution in property law and Birks's "core case" of the claim to recover a mistaken payment, and in contract law.