The Index Librorum Prohibitorum (Latin: Index of Forbidden Books), commonly referred to as the Index, was an official list of publications that the Roman Catholic Church prohibited its members from reading, possessing, or distributing without authorization. Maintained by the Congregation of the Index and later the Holy Office, the Index was in continuous operation from 1559 until its formal abolition in 1966. At its height it contained thousands of titles spanning theology, philosophy, natural science, history, and literature.

The Index functioned as an instrument of doctrinal discipline. Its stated purpose was to protect the faith and morals of Catholic readers by identifying works judged to contain heresy, doctrinal error, moral harm, or content contrary to Church teaching. Works could be placed on the Index by the Congregation of the Index — established by Pope Pius V in 1571 — or by the Holy Office (the Inquisition). Authors could respond to proposed condemnations; in some cases works were listed with corrections required (*donec corrigatur*) rather than outright prohibition. Reading a listed work without a dispensation was held to constitute a sin, and in certain periods could carry ecclesiastical penalties. Dispensations were granted to theologians, scholars, and others with professional need.

The first Roman Index was issued under Pope Paul IV in 1559 and was regarded by many, including some within the Church, as excessively severe. It was revised under the Council of Trent and reissued in 1564 as the Tridentine Index, which established rules — the Regulae — governing the classification of prohibited works. Subsequent editions appeared under numerous popes. The Congregation of the Index was merged into the Holy Office in 1917. The final edition of the Index, the 32nd, was published in 1948. In 1966, the Holy Office — by then renamed the Congregation for the Doctrine of the Faith — announced that the Index no longer had the force of ecclesiastical law, though listed works were described as retaining a moral cautionary character. For a full account of the Index's development across four centuries, see Index of Forbidden Books - History.

The Index encompassed works across a broad range of categories. Theological works by Protestant Reformers, including Martin Luther, John Calvin, and Philipp Melanchthon, were listed in their entirety. Works in natural philosophy listed on the Index include Nicolaus Copernicus's De Revolutionibus Orbium Coelestium (added 1616, removed 1758) and Galileo Galilei's Dialogue Concerning the Two Chief World Systems (1633). Philosophical works by René Descartes, John Locke, Voltaire, David Hume, and John Stuart Mill appeared on the Index, as did works by Niccolò Machiavelli and Giordano Bruno. Literary works were also listed, including titles by Victor Hugo and Gustave Flaubert. The breadth of listed authors reflects the Index's function as a doctrinal instrument that extended well beyond narrowly theological disputes.

The Index operated alongside but was distinct from other Church censorship mechanisms. The //imprimatur// and //nihil obstat// systems governed pre-publication approval for Catholic works. The Inquisition had independent authority to prosecute individuals for heresy, which was a distinct proceeding from mere listing on the Index. Civil authorities in Catholic-majority states sometimes enforced Index prohibitions through secular law, though the degree of enforcement varied widely by jurisdiction and period. The relationship between the Index and civil censorship in early modern Europe is treated in Index of Forbidden Books - History.

The Index was contested throughout its history, both from within the Church and from external critics. Some Catholic scholars and bishops objected to particular listings or to the process by which works were condemned; others argued the Index was insufficiently comprehensive. Protestant thinkers rejected the Index as an illegitimate exercise of ecclesiastical authority incompatible with freedom of conscience. Enlightenment critics viewed it as an emblem of intellectual suppression. Whether the Index constituted a meaningful impediment to the circulation of ideas, or whether its enforcement was too inconsistent to achieve that effect, is a matter addressed in Index of Forbidden Books - Effectiveness Viewpoint. The question of the Index's relationship to the development of science — particularly in the cases of Copernicus and Galileo — is addressed in Heliocentrism - Historical Catholic Viewpoint. Contemporary Catholic assessments of the Index's legacy vary; see Index of Forbidden Books - Catholic Retrospective Viewpoint.

The 1966 announcement ending the Index's binding authority was interpreted variously as an acknowledgment that the mechanism had become unworkable, as a gesture of openness following the Second Vatican Council, or as a substantive shift in the Church's relationship to intellectual freedom. The Index is frequently cited in broader discussions of censorship, religious authority, and the history of science. Its legacy is evaluated differently by Catholic traditionalists, Catholic reformers, secular historians, and advocates of press freedom; the range of interpretations is addressed in Index of Forbidden Books - Catholic Retrospective Viewpoint and Index of Forbidden Books - Secular Historical Viewpoint.

• Heliocentrism
• Heliocentrism - Historical Catholic Viewpoint
• Heliocentrism - Galileo
• Heliocentrism - History
• Inquisition