The Church has always prohibited evil books from the time of the Apostles. Saint Paul oversaw the burning of millions of dollars of evil books as reported in Acts. On the 25th of January, 1897, the late Pope, Leo XIII., gave to the world an important letter, entitled, Officiorum ac Munerum," dealing with the Legislation of the Index and its gravity and far-reaching influence entitle it to more wide-spread knowledge than it has hitherto attained Though intended primarily for the direction of Bishops an other ecclesiastical functionaries, occupying positions o responsibility, yet its import affects tho public at large t so great an extent that its contents deserve to be place within the reach of all. Hitherto little has been done I this country to secure this end. Commentaries have alread appeared in most European languages; but until the issu of the present work, nothing had come before the public I an English dress to bring the people of these countries I touch with the most recent legislation on the Index The Pontifical letter referred to was intended, by it illustrious author, to form a preface to certain Rules which had been adopted and approved by the Congregation o the Index in reference to the Prohibition and Censorshi of Books; and these two documents, together with the Sollicita ac Provida of Benedict XIV., which also finds place in the present publication, bring the Church's legislation on the Index up to date. The scope of the entire Constitution, and, therefore, of the present work, which purports to be a scientific commentary upon it, will be best understood from a brief analysis of the treatment followed in both. At a time when the world is flooded with pernicious literature, and the poison not alone of immorality, but of unbelief, is being slowly but surely instilled into the public mind, the importance of a book dealing with such a subject cannot be exaggerated. The evil is prevalent in Europe, America, and Australia, and, no doubt, much of the danger to Church and State, that is at present menacing most of our European countries, springs largely from the unbridled licence of the Press. Even here in Ireland, there are grave apprehensions entertained for the future from the same source. That the Bishops of the country are not unmindful of this danger is evidenced from the fact that in the Appendix to the Acta et Decreta of the Synod of Maynooth (1900), just published, the legislation dealt with in this work is given in full. Let us hope that tho present book will prove helpful to both the Clergy and laity of the country who wish to place themselves in touch with the latest legislation on the Prohibition of Books; the Censorship of the Press, and the Rules of the Index.