This study is concerned with the missiological thinking as expressed in the conferences and assemblies of the International Missionary Council and the World Council of Churches. It helps us to get an overview of the twentieth century ecumenical missiology. The missiological discussions in these conferences and assemblies are treated under five major topics: Church and Mission; Mission and Unity; World, Mission and Church; The Kingdom of God and Mission; and Mission and The World of Religions and Cultures. If mission as conquest was a dominant note at the beginning of this century, mission as living in dialogical relationship is the emphasis at the end of the century.