Indulgentiarum Doctrina is an apostolic constitution about indulgences issued by Pope Paul VI on 1 January 1967. It responds to suggestions made at the Second Vatican Council, it substantially revised the practical application of the traditional doctrine relating to indulgences. The title is taken from the opening words of the original Latin text.

According to the Catechism of the Catholic Church, "The forgiveness of sin and restoration of communion with God entail the remission of the eternal punishment of sin, but temporal punishment of sin remains."

Paul VI explained that sin brings punishments inflicted by God's sanctity and justice, which must be expiated either here on earth or else in the life to come. "These punishments are imposed by the just and merciful judgment of God for the purification of souls, the defense of the sanctity of the moral order and the restoration of the glory of God to its full majesty." Such expiation generally takes the form of penance, traditionally described as prayers, fasting, and alms, but also includes works of mercy and charity.

"That punishment or the vestiges of sin may remain to be expiated or cleansed and that they in fact frequently do even after the remission of guilt is clearly demonstrated by the doctrine on purgatory. In purgatory, in fact, the souls of those 'who died in the charity of God and truly repentant, but before satisfying with worthy fruits of penance for sins committed and for omissions' are cleansed after death with purgatorial punishments".

The document stressed that the Church's aim was not merely to help the faithful make due satisfaction for their sins, but chiefly to bring them to greater fervour of charity. For this purpose, Paul VI decreed that partial indulgences, previously granted as the equivalent of a certain number of days, months, quarantines, or years of canonical penance, simply supplement, and to the same degree, the remission that those performing the indulgenced action already gain by the charity and contrition with which they do it. "For all men who walk this earth daily commit at least venial sins; thus all need the mercy of God to be set free from the penal consequences of sin."

The abolition of the classification by years and days made it clearer than before that repentance and faith are required not only for remission of eternal punishment for mortal sin but also for any remission of temporal punishment for sin. "Indulgences cannot be gained without a sincere conversion of outlook and unity with God".

An indulgence is the remission before God of the temporal punishment due sins already forgiven as far as their guilt is concerned. "The aim pursued by ecclesiastical authority in granting indulgences is not only that of helping the faithful to expiate the punishment due to sin but also that of urging them to perform works of piety, penitence and charity—particularly those which lead to growth in faith and which favor the common good."

An indulgence is partial or plenary accordingly, as it removes either part or all of the temporal punishment due sin. Indulgences can always be applied to the dead by way of suffrage.