INDEX LIBRORUM PROHIBITORUM, the title of the official list of those books which on doctrinal or moral grounds the Roman Catholic Church authoritatively forbids the members of its communion to read or to possess, irrespective of works for bidden by the general rules on the subject. The earliest known instance of a list of proscribed books being issued with the authority of a bishop of Rome is the Notitia librorum apocry phorum qui non recipiuntur, the first redaction of which, by Pope Gelasius (494), was subsequently amplified on several occasions. The document is for the most part an enumeration of apocryphal gospels and acts. One of the functions of the In quisition when it was established was to exercise a rigid censor ship over books put in circulation. The majority of the con demnations were at that time of a specially theological character. With the discovery of the art of printing, and the wide and cheap diffusion of all sorts of books which ensued, the need for new pre cautions against heresy and immorality in literature made itself felt ; more than one pope (Sixtus IV. in 1479 and Alexander VI. in 15o1) gave special directions regarding the growing freedom of the printing press, and in 1515 the Lateran council attempted to forbid the printing of any book without previous examination by the ecclesiastical authority. The Council of Trent in its fourth session, April 1546, forbade the sale or possession of any anony mous religious book which had not previously been seen and ap proved by the ordinary; in the same year the university of Lou vain, at the command of Charles V., prepared an "Index" of per nicious and forbidden books, a second edition of which appeared in 155o. In 1557, and again in 1559, Pope Paul IV., through the Inquisition at Rome, published what may be regarded as the first Roman Index in the modern ecclesiastical use of that term (Index auctorum et librorum qui tanquam haeretici aut suspecti aut per versi ab Officio S. R. Inquisitionis reprobantur et in universa Christiana republica interdicuntur) . In this we find the three classes which were to be maintained in the Trent Index: authors condemned with all their writings; prohibited books, the authors of which are known; pernicious books by anonymous authors. At the i8th session of the Council of Trent (February 1562), in consideration of the great increase in the number of suspect and pernicious books, and also of the inefficacy of the many previous "censures" which had proceeded from the provinces and from Rome itself, a commission was appointed to inquire into these "censures," and to consider what ought to be done in the cir cumstances. The result of its labours was handed over to the pope to deal with as he should think proper. In the following March accordingly were published, with papal approval, the Index librorum prohibitorum, which continued to be reprinted and brought down to date, and the "Ten Rules" which regulated the matter until the pontificate of Leo XIII. (1897) . The business of condemning pernicious books and of correcting the Index to date has been since the time of Pope Sixtus V. in the hands of the "Congregation of the Index," which consists of several cardin als, and more or less numerous "consultors" and "examiners of books." With the alteration of social conditions, however, the Rules of Trent ceased to be entirely applicable. Their application to publications which had no concern with morals or religion was no longer conceivable ; and, finally, the penalties called for modi fication. Already, at the Vatican Council, several bishops had submitted requests for a reform of the Index, but the Council was not able to deal with the question. The reform was accom plished by Leo XIII., who, on the 25th of January 1897, pub lished the constitution Officiorum, in 49 articles. In this consti tution, although the writings of heretics in support of heresy are condemned as before, those of their books which contain nothing against Catholic doctrine or which treat other subjects are per mitted. Editions of the text of the Scriptures are permitted for purposes of study ; translations of the Bible into the vulgar tongue have to be approved, while those published by non Catholics are permitted for the use of scholars. Obscene books are forbidden ; the classics, however, are authorized for educa tional purposes. Books and newspapers which outrage God and sacred things, which propagate magic and superstition, or which are pernicious to society are forbidden. Permissions to read pro hibited books are given by the bishop in particular cases, and in the ordinary course by the Congregation of the Index. The con stitution then proceeds with the censorship of books. The ex amination of the books is entrusted to censors, who have to study them without prejudice ; if their report is favourable, the bishop gives the imprimatur. All books concerned with the religious sciences and with ethics are submitted to preliminary censorship, and in addition to this ecclesiastics have to obtain a personal authorization for all their books and for the acceptance of the editorship of a periodical. The penalty of excommunication ipso facto is only maintained for reading books written by heretics or apostates in defence of heresy, or books condemned by name under pain of excommunication by pontifical letters. The con stitution also prescribed a revision of the catalogue of the Index. The new Index, which omits works anterior to 1600 as well as a great number of others included in the old catalogue, appeared in 1900. The encyclical Pascendi of Pius X. (8th September 1907) made it obligatory for periodicals amenable to the eccle siastical authority to be submitted to a censor for report.