In laws of equity, unjust enrichment occurs when one person is enriched at the expense of another in circumstances that the law sees as unjust. Where an individual is unjustly enriched, the law imposes an obligation upon the recipient to make restitution, subject to defenses such as change of position. Liability for an unjust (or unjustified) enrichment arises irrespective of wrongdoing on the part of the recipient. The concept of unjust enrichment can be traced to Roman law and the maxim that "no one should be benefited at another's expense": nemo locupletari potest aliena iactura or nemo locupletari debet cum aliena iactura.

The law of unjust enrichment is closely related to, but not coextensive with, the law of restitution. The law of restitution is the law of gain-based recovery. It is wider than the law of unjust enrichment. Restitution for unjust enrichment is a subset of the law of restitution in the same way that compensation for breach of contract is a subset of the law relating to compensation.

History.

Roman law.

In civil law systems, unjust enrichment is often referred to as unjustified enrichment. Its historical foundation of enrichment without cause can be traced back to the Corpus Iuris Civilis. While the concept of enrichment without cause was unknown in classical Roman law, Roman legal compilers eventually enunciated the principle of unjustified enrichment based on two actions of the classical Roman period—the condictio and the actio de in rem verso.

The condictio authorized recovery by the plaintiff of a certain object or money in the hands of the defendant. The defendant was considered a borrower who was charged with returning the object or money. For the actio de in rem verso, the plaintiff bore the burden of specifying the cause for his demand, namely, demanding the restitution of assets that had exited the plaintiff's patrimony and entered the defendant’s patrimony through the acts of the defendant’s servants.

The coherent concept of unjustified enrichment, then appeared in the Justinian Code, based on Roman pragmatism with equitable considerations and moral principles of Greek philosophy. In the Justinian Code, condictiones were grouped into categories, such as when the plaintiff had given a thing or money:

1. in contemplation of a future result that did not follow.

2. for a reason disapproved by law or repugnant to public policy.

3. by mistake because payment was not actually due; or

4. without a good reason for the transaction.