The contribution starts from the Church’s fundamental declarations on its action in the world, analysing their magisterial and canonical foundations, and then dwells on the subject of the exercise of the right of legation and the Holy See’s role as mediator in conflicts between third parties. It then proceeds with an investigation into the contractual activity of the Catholic Church itself in its historical development, focusing in particular on the legal significance of Concordats and the globalisation of concordat law. Finally, in conclusion, the research pays due attention to «open» and «enlarged» bilateralism