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 place. The author intends to show that the special nature of unjust enrichment is determined first and foremost by the fulfilment of the conditions for its occurrence. These conditions include: an individual’s gain, benefiting at the expense of another person, the relationship between enrichment and impoverishment and the lack of legal grounds for enrichment; it should be added, however, that the Civil Code does not speak of the last of the listed conditions. The knowledge or the will of the enriched person will not be considered such conditions, though. The liability of the enriched person in such circumstances will occur regardless of the cause of the person’s gain.