Unjust Enrichment

Unjust enrichment is a doctrine in both civil law and common law systems holding that a person who receives a benefit at another's expense without legal justification must return the value of that benefit. It functions as an independent source of obligation alongside contract and tort, and the remedy it generates is called restitution.

Most jurisdictions analyze unjust enrichment through three or four elements: (1) the defendant was enriched; (2) the enrichment came at the claimant's expense; (3) the enrichment was unjust — meaning there was no contractual, statutory, or other legal basis for retaining it; and (4) in some systems, the absence of a defense such as change of position or bona fide purchase.

In the common law tradition, the modern English law of restitution was crystallized by the House of Lords in Lipkin Gorman v Karpnale Ltd [1991] 2 AC 548, which recognized unjust enrichment as a distinct cause of action. Subsequent cases such as Kleinwort Benson Ltd v Lincoln City Council [1999] 2 AC 349 extended recovery to payments made under mistake of law. In the United States, the doctrine is set out in the Restatement (Third) of Restitution and Unjust Enrichment (2011).

Civil law systems codify the principle directly — for example, §812 of the German Bürgerliches Gesetzbuch (BGB) on ungerechtfertigte Bereicherung, and Articles 1303–1303-4 of the French Civil Code following the 2016 reform of contract law.

In international law and investor-state arbitration, unjust enrichment has been invoked as a general principle of law under Article 38(1)(c) of the ICJ Statute. Tribunals have referenced it in cases involving expropriation compensation and state succession, though it is more often a supporting argument than a primary basis for recovery. The principle also appears in debates over reparations, sanctions windfalls, and the return of frozen sovereign assets.