This chapter discusses the origin and work of the Pontifical Council for Promoting Christian Unity (PCPCU) as the major instrument of the Holy See for ecumenical matters. Established in 1960, it was first an instrument of the Second Vatican Council, assisting the Council especially, but not exclusively, in decisions relating to ecumenism and inter-religious matters. Constituted part of the Roman Curia after Vatican II, it has continued in light of the Council’s Decree on Ecumenism to promote the goal of Christian unity by engaging in ecumenical dialogue and collaboration with other Christians and ecumenical organizations, by advancing ecumenical formation and education within the Catholic Church, and by cultivating, with ecumenical partners, the reception of achievements resulting from dialogue. Related to the PCPCU, though with different goals, is the Holy See’s Commission for Religious Relations with the Jews, tasked with implementing Vatican II’s decree, Nostra Aetate, n. 4.