Summary

Abstract Restitution has been thought to be reducible to a single ‘unjust enrichment’ principle. This in turn has been divided into four distinct questions. Is the defendant enriched? Is the enrichment at the expense of the plaintiff? Is the enrichment unjust? Does the defendant have any defence? These questions have been treated as independent of one another, as constituent parts of a cause of action independent of other areas of law. The negative purpose of this work is to show that this is misleading. The positive purpose is to distinguish the different principles governing the various claims there are. In particular, it looks at: payments made that are not due, and cognate cases such as the rendering and acceptance of a service that is unjustified; a performance rendered conditionally under an agreement, where the condition has failed; the discharge of an obligation properly borne by another; why claims for salvage, general average, and other examples of ‘necessitous intervention’ are justifiable; how the right to things may be lost through accession, mixture, specification, and conveyance; when and why obligations to (re-)transfer rights give rise to constructive and resulting trusts, and how equitable tracing and subrogation can be explained; the limited circumstances in which a gain-based remedy for wrongdoing is possible; when and why the different defences applicable to the different kinds of claim arise; and the impact of illegality on claims for restitution.