The second Vatican Council and the Synod of Bishops are connected by the fact that Pope Paul VI created the latter during the Council in order to keep alive the spirit of collegaiality experienced by the Council Fathers. This link has also been evident in the themes chosen for each Synod since they have provided opportunities to unpack and apply the theaching of the documents of the Second Vatican Council to various aspects of the Church’s life. One topic that flows from the council through each Synod is the Church’s teaching on Ecumenism. Although much has been written about the Council and the Synods in the context of Ecumenism, one component that has not yet received ample attention is the partciipation of the non-Catholic Christian observers and guests at the Second Vatican Council and the fraternal delegates at the Synods of Bishops from various Churches, Ecclesial Communities and Ecclesial Organizations. The positive experience of the participation of the observers and guest at the Council eventually prompted the Catholic Church to welcome observers (later called fraternal delegates) to various assemblies of the Synod of Bishops. Therefore, the purpose of this dissertation is to provide a fuller understanding of these important ecclesial events and to analyze the implications it has for the understanding of the Chirches’s Magisterium.