The institute of unjust enrichment is based on the principle, that no one should be benefited at another’s expense in circumstances that the law sees as unjust. The concept has its roots in Roman law, and it is based on the old maxim of Pomponius. In Roman law, the institute of unjust enrichment was not known in the sense, as we know it today. It was not a general source of obligations, but rather linked with four specific legal institutes. The most important of all was the so-called condictio which could have been used in cases when someone was enriched at another’s expenses. In the period of post-classical law, individual actions were classified, expanded and generalized in use. The most important novelty was that, in certain cases, the time of filing the claim became extremely important. The claim (typical example is so-called actio de in rem verso) already referred to what the enriched still possessed at the time when the action was brought, and not to the return of what he had received. Consequently, two systems developed in dealing with unjust enrichment: the pluralistic and the unitary legal concept. Slovenia has adopted the unitary system with a general rule, which is used in all cases where an unjustified transfer of assets occurs.
In the period of Roman law, the institute of unjust enrichment was only evolving, which makes the difference between the one and the other. Unlike the Roman legal system, the Slovenian law applies the enrichment principle and not the principle of restitution. In addition, the rules on return are much more regulated (when restitution in kind is required and when monetary restitution is appropriate, when the return of the benefit of using things is required, and when the recipient can make a counter-claim); the same applies to the statute of limitation.