The missionary activity of the Church in the world today is subject to many pressures and hostile forces that have developed in the period that followed the First World War: the enormous population increase which has not been matched by a proportionate increase in the number of Catholics, political and nationalist hostility in the newly-independent countries, renewed opposition from non-Christian religions such as Buddhism and Islam, and the spread of Communism. This book describes the dimensions of these new factors and the way in which the Church is countering them together with the advances made in the development of the missions under the guidance of the four great modern popes, Benedict XV, Pius XI, Pius XII and John XXIII. René Millot also describes the structure of modern missionary organization, the development of native clergies and hierarchies, the role of the catechist, and the social, medical and educational work carried out by the missions. A survey of missions throughout the world from Finland to Japan, providing detailed facts and figures, is followed by a general conclusion in which future trands and challenges are discussed and analysed.