In the teaching of the Catholic Church, an indulgence (Latin: indulgentia, from indulgeo, 'permit') is "a way to reduce the amount of punishment one has to undergo for (forgiven) sins". The Catechism of the Catholic Church describes an indulgence as "a remission before God of the temporal punishment due to sins whose guilt has already been forgiven, which the faithful Christian who is duly disposed gains under certain prescribed conditions…"

The recipient of an indulgence must perform an action to receive it. This is most often the saying (once, or many times) of a specified prayer, but may also include a pilgrimage, the visiting of a particular place (such as a shrine, church, or cemetery), or the performance of specific good works.

Indulgences were introduced to allow for the remission of the severe penances of the early church and granted at the intercession of Christians awaiting martyrdom or at least imprisoned for the faith. The Catholic Church teaches that indulgences draw on the treasury of merit accumulated by Jesus's death on the cross and the virtues and penances of the saints. They are granted for specific good works and prayers in proportion to the devotion with which those good works are performed or prayers recited.

By the late Middle Ages, indulgences were used to support charities for the public good, including hospitals. However, the abuse of indulgences for almsgiving, so that they became a method of moneyraising or ignored the requirements for contrition or charity, had become a serious problem which the church recognized but was unable to restrain effectively. Abuses in the practise and teaching on indulgences were, from the beginning of the Protestant Reformation, a target of attacks by Martin Luther and other Protestant theologians. Eventually, the Catholic Counter-Reformation curbed the abuses of indulgences, but indulgences continue to play a role in modern Catholic religious life, and were dogmatically confirmed as part of the Catholic faith by the Council of Trent. In 1567, Pope Pius V forbade tying indulgences to any financial act, even to the giving of alms. Reforms in the 20th century largely abolished the quantification of indulgences, which had been expressed in terms of days or years. These days or years were meant to represent the equivalent of time spent in penance, although it was widely mistaken to mean time spent in Purgatory. The reforms also greatly reduced the number of indulgences granted for visiting particular churches and other locations.^{[citation needed]}

Catholic teaching states that when a person sins, they acquire the liability of guilt and the liability of punishment. A mortal sin, one that is grave or serious in nature and is committed knowingly and freely, is considered to be an active refusal of communion with God, and to separate a person from Him to the end of suffering the eternal death of hell as an effect of this rejection, a consequence known as the "eternal punishment" of sin. The Sacrament of Penance removes this guilt and the liability of eternal punishment related to mortal sin.

The forgiveness of sin and restoration of communion with God entail the remission of the eternal punishment of sin, but the temporal punishment of sin remains. An example of this can be seen in 2 Samuel 12, when, after David repents of his sin, the prophet Nathan tells him that he is forgiven, but, "Thus says the Lord God of Israel:...Now, therefore, the sword shall never depart from your house, because you have despised me and have taken the wife of Uriah to be your wife."

In addition to the eternal punishment due to mortal sin, every sin, including venial sin, is a turning away from God through what the Catechism of the Catholic Church calls an "unhealthy attachment to creatures", an attachment that must be purified either here on earth, or after death in the state called purgatory. "The process of sanctification and interior renewal requires not only forgiveness from the guilt (culpa) of sin, but also purification from the harmful effects or wounds of sin." This purification process gives rise to "temporal punishment", because, not involving a total rejection of God, it is not eternal and can be expiated. Catholic teaching states that the temporal punishment of sin should be accepted as a grace, and that the sinner "should strive by works of mercy and charity, as well as by prayer and the various practices of penance, to put off completely the 'old man' and to put on the 'new man'."

The temporal punishment that follows sin is thus undergone either during life on earth or in purgatory. In this life, as well as by patient acceptance of sufferings and trials, the necessary cleansing from attachment to creatures may, at least in part, be achieved by turning to God in prayer and penance and by works of mercy and charity. Indulgences (from the Latin verb 'indulgere', meaning "to forgive", "to be lenient toward") are a help towards achieving this purification.