Index Liekorum (a catalogue of books censured by the supreme authority in the Roman Catholic church as prejudicial to faith and good morals. This catalogue is twofold: that of books absolutely forbidden to be read, Index Librorum Prohibitorum; and that of books forbidden only until they are expurgated or corrected by their author, Index Librorum Ex-purgandorum. It is published by the "Congregation of the Index," composed of cardinals designated by the pope, with a secretary, who is by right a Dominican monk, and a body of examining theologians, usually belonging to the other religious orders, and called the consultum. To the consultum are submitted the suspected books, and they report back to the cardinals, who hold their deliberations either in presence of the pope, or in the house of the senior cardinal of their number. All books that treat ex professo of faith or morals, of ecclesiastical discipline, or of civil society, fall within their jurisdiction; and the judgment of the congregation either suppresses the book altogether, or commands it to be corrected, or permits it to be read under certain conditions, or by a certain class of persons only.

The first official index or catalogue was prepared by the inquisition at Rome, and published in 1557 by order of Pope Paul IV. This, enlarged and reduced to a regular form by a committee of the council of Trent, was published anew in 1564, with the sanction of Pius IV., and enlarged by Clement VIII. in 1595. The latest official edition is that of 1819. The index is continually enlarged by the supplementary lists of each year. The congregation of the index originated with the council of Trent under Pius IV., and its official establishment is due to Pius V.