Unjust enrichment describes a situation where one party is enriched at another’s expense in circumstances the law considers unjust, and where restitution is ordered to reverse that enrichment. It is a core principle in the law of restitution rather than a single codified statutory concept, and is primarily developed through case law across the UK and Ireland. In England and Wales and Northern Ireland, courts commonly analyse unjust enrichment using four elements: (1) the defendant’s enrichment; (2) at the claimant’s expense; (3) an unjust factor (such as mistake, duress, failure of consideration or undue influence); and (4) the absence of a relevant defence (for example change of position). In Scotland, unjustified enrichment is recognised as a distinct obligation, historically structured around specific “restitutionary” and “recompense” actions, but now often analysed in similar modern terms. In Ireland, the doctrine closely follows English restitution principles, with Irish case law adapting unjust enrichment to local statutory and equitable contexts. Unjust enrichment claims frequently arise in mistaken payments, failed contracts, overpayments, benefits conferred under void transactions, and misapplied funds, and are a key alternative or supplement to contractual and equitable remedies.