The study proposes an analysis of the lawful juridical fact of the undue payment circumscribed to the sphere of lawful juridical facts, source of the legal civil obligation of restitution, consisting in making a payment, voluntarily, but undue, by mistake. In the introductory part of the paper, we presented the definition of undue payment and the provisions of the Civil Code regarding the lawful juridical fact of undue payment, but also those provisions to which they refer, generally applicable to a "payment". In the second part we analysed the Conditions of undue payment, which must be met cumulatively: a payment must be made, which is not due, in the absence of a debt, by mistake. We pointed out that although the current regulation did not expressly mention the condition that the payment was made in error, in order to be able to justify the action in repetition for the benefit of solvens, the payment must have been made by error. Thus, it is important the so-called „volitional component" with reference to the intention of the solvens to extinguish an existing debt to the accipiens, having the firm conviction that this obligation exists and that the one who receives the payment would really be his creditor. In the hypothesis in which the solvens pays disinterestedly, without pursuing the payment of a debt, his gesture could be interpreted as a liberality, with all the consequences arising from this qualification, including the impossibility of restitution due to the irrevocable nature of the legal operation. The third part of the study was devoted to the Effects of the undue payment, distinct between solvens and accipiens, respectively those towards third parties, occasion to present various hypotheses with the indication of some jurisprudential solutions pronounced in this matter. In the final part, we presented the particularities of the action for restitution with the indication of the owner of the action, the plaintiff solvens against the defendant accipiens, specifying the legal nature and the object of this action, the rules regarding the prescription, cases of inadmissibility and the possible legal solutions, depending on the good or bad faith of accipiens. As a study method, we sought to combine the theoretical analysis with the position of the jurisprudence in this field, considering that this approach is edifying in presenting the specific features of the undue payment, in particular, in comparison with the other lawful juridical facts as sources of obligations, fraudulent management and enrichment without a just cause. Thus, we went through valuable works from our classical doctrine, but also from the contemporary one, in the comments of the texts of the current Civil Code, identifying the published and commented jurisprudential solutions. [ABSTRACT FROM AUTHOR]