The revived interest in Scholastic philosophy, which received such a powerful impetus from the publication of the "Aeterni Patris" by Leo XIII, in 1879, far from abating, continues to increase. In ecclesiastical circles the works of Saint Thomas are of prime importance, especially since Pope Pius X. in his Letter against Modernism (1907) and in the "Doctoris Angelici" (1914), decreed that Scholastic philosophy according to the mind and method of St. Thomas should be the basis for the theological studies which are to be a safeguard and bulwark against modernistic errors and tendencies. The new Code of Canon Law imposes on all professors of philosophy and theology the obligation of adhering religiously to the doctrine and principles of St. Thomas. The Angelic Doctor was the greatest of many men who cultivated reason and used it in the defence and explanation of Christian truth. Many volumes would be required to give a comprehensive review of thirteenth century conditions and of his influence on medieval thought, and the following pages do not pretend to give such a complete and comprehensive review. It is hoped, however, that they will serve to explain, in a brief and summary manner, the influence of St. Thomas on medieval philosophy, thereby creating a desire for deeper study of that important period. For those who intend to pursue ecclesiastical studies some knowledge of conditions prevailing in the Scholastic period is essential, since without this knowledge they will find it impossible properly to appreciate the work of the great Scholastic doctors. All cannot be told in one book, but even a short history of that period, with mention of the principal errors which the Scholastics were called upon to combat, will add to our gratitude for the services rendered by those enlightened and valiant defenders of the Faith. The matter is treated in a popular way, so that from the sketches given even the ordinary reader can form a fairly accurate conception of the position that St. Thomas holds in the history of medieval philosophy. For the benefit of those who may wish to make a deeper study of this subject there is added a bibliography, which will be found especially helpful to beginners.