Donation in contemplation of death was one of the basic examples of datio ob rem which is an essential element of condictio causa data causa non secuta, which is the Roman root of the contemporary concept of performance rendered for an intended purpose that has not been achieved within the meaning of art. 410 § 2 of the Polish civil code of 1964. In this article the legal nature of this donation and various forms of its application are described. Particular attention is paid to the purpose of the donor’s performance in relation to the general idea of “giving on purpose” (datio ob rem) in Roman law. Moreover, the evolution of donatio mortis causa towards its assimilation with the institution of Roman inheritance law, especially legacies, is shown.