Since the age of Luther and the Protestant Revolt, indulgences have been to non Catholics one of the least understood and most criticized institutions of the Catholic Church. Some of the misconception is undoubtedly due to the fact that the writings of Catholic historians, lay or cleric, too generally ignore the point of view of the intelligent non-Catholic. Some of it, equally without doubt, is due to the failure of non-Catholic secular historians to avail themselves fully even of the means at hand to understand the ecclesiastical viewpoint. In the present essay, which Fr. Ross has put into such straightforward English, Dr. Paulus has not concerned himself with the doctrine of indulgences, and for that reason, perhaps, his book will make a wider appeal to the non-Catholic mind. Frequently the indulgence was merely a permit to commute one form of penance into another, and Dr. Paulus has pointed out the social significance of this fact. Fasting or some other such penance was changed into a contribution of money or service to some useful public undertaking, and Dr. Paulus presents an impressive list of these works due to a commutation of penances, or indulgences. The list includes the construction and maintenance of churches, schools, hospitals, and other charitable institutions; bridges, dams, roads, harbors and fortifications, and the stimulation of such important social movenlents as Crusades and the Truce of God. This latter was the first really effective step taken to end the almost universal petty warfare, pillage, and banditry of the early Middle Ages, and of itself would give importance to this feature of indulgences. In making this contribution to Church history easily accessible in English, Fr. Ross has rendered a service of genuine value.