Although the modern ecumenical movement has existed for more than a century, the Roman Catholic Church’s involvement began only some 50 years later. In fact, 2014 commemorates only the 50th anniversary of the promulgation of Unitas redintegratio, Vatican II’s Decree on Ecumenism. This article traces the ecumenical movement from its beginnings, focusing on the contributions of the World Council of Churches and the progress made by Roman Catholicism. It concludes by pointing to issues which World Christianity is exploring as the ecumenical agenda for the 21st century.