The author highlights main doctrinal theories on legal nature of concordats, the specific features of their subject and the current understanding of their nature in contemporary international law. It is emphasized that concordats do not fall within the standard aims and content of ordinary international treaties and refer to matters of so called “mixed competence”: international and domestic, spiritual and temporal. It is also mentioned that in strict understanding of the provisions of the Vienna Convention on the Law of Treaties (1969) concordats do not fall within the scope of the Convention. The aforesaid, however, does not vitiate a concordat or prevent the parties from subjecting it to the Vienna Convention. A brief characteristic of the Lateran concordat of 1929 is given. Concordat is defined as an international legal agreement between the Holy See and a particulate state with the aim of regulating the bilateral relations between its parties, determining the legal status and terms of activity of the Catholic Church within the state and guaranteeing the Church’s interrelation with the Holy See.