The purpose of the article is to discuss unjust enrichment as a source of obligations, already known in Roman law, which dealt with the most important problems related to the matter in question and ways to solve those problems in practice. The author, aware of the complexity of the issues related to unjust enrichment and the problems that may arise from the application of this institution, will focus on the presentation of the definition and the characteristics of unjust enrichment as a source of obligations. These characteristics are covered only in Article 405 of the Civil Code, but the apparently obvious regulation includes some unnamed elements that need to be taken into account when deciding whether unjust enrichment has actually taken...