Concept
The English law of unjust enrichment is part of the English law of obligations, along with the law of contract, tort, and trusts. The law of unjust enrichment deals with circumstances in which one person is required to make restitution of a benefit acquired at the expense of another in circumstances which are unjust. The modern law of unjust enrichment encompasses what was once known as the law of quasi-contract. Its precise scope remains a matter of controversy. Beyond quasi-contract, it is sometimes said to encompass the law relating to subrogation, contribution, recoupment, and claims to the traceable substitutes of misapplied property. English courts have recognised that there are four steps required to establish a claim in unjust enrichment. If the following elements are satisfied, a claimant has a prima facie right to restitution: the defendant has been enriched; this enrichment is at the claimant's expense; this enrichment at the claimant's expense is unjust; and there is no applicable bar or defence. The law of unjust enrichment is among the most unsettled areas of English law. Its existence as a separate body of law was only explicitly recognised in 1991 in Lipkin Gorman v Karpnale Ltd. While the law has rapidly developed over the last three decades, controversy continues over the precise structure, scope and nature of the law of unjust enrichment. The notion of an obligation to make restitution of benefits received at another's expense can be traced back to Roman law. Its history in English law can be traced to the form of action known as indebitatus assumpsit. From this action came the 'common money counts'. Of present relevance are the following: an action for money had and received to the plaintiff's use; an action for money paid to the defendant's use; quantum meruit (that is, a claim for reasonable remuneration for services provided by the plaintiff to the defendant); and quantum valebat (that is, a claim for the reasonable value of goods provided by the plaintiff to the defendant).