This book is a study on the ideas of Thomas Aquinas, one of the greatest philosophers and theologians of all time, on the Catholic doctrine of Immaculate Conception. Though Thomas had denied the idea of Immaculate Conception of Mary which was not yet declared a dogma then, it has to be understood in the historical viewpoint and the motivations behind such a stand from the prince of theologians. The book tries to show the real idea of Thomas on the topic with a philosophical understanding of his teaching on the same especially in the light of his much celebrated work Summa Theologiae.