(Latin: “Index of Forbidden Books”), list of books once forbidden by Roman Catholic church authority as dangerous to the faith or morals of Roman Catholics. Publication of the list ceased in 1966, and it was relegated to the status of a historic document.

Compiled by official censors, the Index was an implementation of one part of the teaching function of the Roman Catholic church: to prevent the contamination of the faith or the corruption of morals through the reading of theologically erroneous or immoral books. It was not, therefore, equivalent to the total legislation of the church regulating reading by Roman Catholics; nor was it ever a complete catalog of forbidden reading. Until 1966, canon law prescribed two main forms of control over literature: the censorship of books by Roman Catholics in advance of publication, in regard to matters of faith and morals (a practice still followed); and the condemnation of published books that were judged to be harmful. The works appearing on the Index are only those that ecclesiastical authority was asked to act upon.

The origin of the church's legislation concerning the censorship of books is unclear, but books were a source of concern as early as the scriptural account of the burning of superstitious books at Ephesus by the new converts of St. Paul (Acts 19:19). The decree of Pope Gelasius I about 496, which contained lists of recommended as well as banned books, has been described as the first Roman Index. The first catalog of forbidden books to include in its title the word index, however, was published in 1559 by the Sacred Congregation of the Roman Inquisition (a precursor to the Congregation for the Doctrine of the Faith). The last and 20th edition of the Index appeared in 1948. The list was suppressed in June 1966.