Annulment is a legal procedure for declaring a marriage null and void. Unlike divorce, it is retroactive: an annulled marriage is considered never to have existed.

In strict legal terminology, annulment refers only to making a voidable marriage null; if the marriage is void ab initio, then it is automatically null, although a legal declaration of nullity is required to establish this. The process of obtaining such a declaration is similar to the annulment process. Generally speaking, annulment, despite its retrospective nature, still results in any children born being considered legitimate in the United States and many other countries.

Annulment in the Catholic Church

main: Annulment (Catholic Church)

In the Roman Catholic Church, a marriage is considered to be a valid contract entered into between two willing parties, and ratified by Divine sanction. In simplest terms, it is necessary that it be marriage that is contracted, that it actually be contracted (i.e., a valid ceremony/contract be performed), and that both parties enter willingly into the contract. If any of these conditions lack, then the marriage is not contracted, Divine sanction is not obtained, and there is in actual (and religious) fact no marriage. An annulment is a finding later that there was no actual marriage contracted in God's eyes, and therefore no marriage in reality (from the religious point of view), regardless of civil ordinance or appearance to humans.

Therefore, an annulment of a marriage is much more analogous to a finding that a contract of sale was invalid, and hence, that the property for sale must be considered to have never legally transferred possession, than analogous to a divorce, which is more like returning the property after a consummated sale.

These four preconditions give rise to the common fourfold classifications for bases of annulment, defect of form, defect of contract, or unwilling or unable parties.

The contract is defective in form if the marriage ceremony is invalid, such as the case of two Catholic persons being married outside of the Catholic Church.

The contract is defective of contract if it was not a marriage that was contracted, such as if there was a defect of intent on either side. This can occur if either party lacked the intent to enter into a lifelong, exclusive union, open to reproduction.

If either party was coerced, they lacked willingness, and therefore lacked intent.

If either party was married to another, they were unable to enter into the contract. Also, certain relationships of blood render the parties unable to enter into contract. Also, parties of the same gender are unable to enter into contract.

Some accuse the Catholic Church of hypocrisy for teaching that all marriages are permanent but providing the means of annulment. The Church attempts to reconcile these two seemingly opposing ideas by understanding that a "Declaration of Nullity" is not a dissolution of a marriage, but rather to determine whether a marriage was a sacrament (valid) or contrary in some way to Divine Law as understood by the Catholic Church or contrary to the prescriptions of canon law regulating marriage. While some may try to use an annulment to get around the "no divorce" rule, that is not the reason the Church gives for the availability of annulment. According to the Church, an annulment affirms the Scriptural basis of divorce and at the same time affirms that in a true marriage, a man and a woman become one flesh before the eyes of God. The Church's teaching on marriage is that it is a Sacrament and that it is only validly contracted by the two individuals, so questions may arise as to whether that person is able to contract a valid marriage. In the Western tradition, the ministers of the marriage are the two individuals themselves, and the priest is a witness for the Church.