Annulment is a legal procedure for declaring a marriage null and void. Annulment differs from divorce where the court ends an otherwise legal marriage on a specific date.

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.

Grounds for annulment

Grounds for a marriage being voidable or void ab initio vary in different legal jurisdictions, but are typically limited to fraud, bigamy, and mental incompetence including that:

1. Either spouse was already married to someone else at the time of the marriage;
2. Either spouse was too young to be married, or too young without required court or parental consent (unless, in some cases, where the marriage continued beyond the age of consent, which validates it);
3. Either spouse was under the influence of drugs or alcohol at the time of the marriage;
4. Either spouse was mentally incompetent at the time of the marriage;
5. If the consent to the marriage was based on fraud or force;
6. Either spouse was physically incapable to be married (typically, inability to have sexual intercourse which persists) at the time of the marriage;
7. The marriage is prohibited by law due to the relationship between the parties. This is the "prohibited degree of consanguity," or blood relationship between the parties. The most common legal relationship is 2nd cousins; 1st cousins are generally not legal.
8. Prisoners sentenced to a term of life imprisonment may not marry.

Annulment in the Catholic Church

In the case of the Catholic Church, annulment does not mean the same thing as divorce. Some accuse the Catholic Church of hypocrisy for preaching that all marriages are permanent but providing the means of annulment. The church reconciles these two seeming 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. 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.

For this reason (or for other reasons that render the marriage null and void),  the church, after an examination of the situation by the competent ecclesiastical tribunal, can declare the nullity of a marriage, i.e., that the marriage never existed. In this case the contracting parties are free to marry, provided the natural obligations of a previous union are discharged. -Catechism of the Catholic Church #1629

A reason for annulment is called an diriment impediment to the marriage. Prohibitory impediments make entering a marriage wrong but do not invalidate the marriage, such as being betrothed to another person at the time of the wedding; diriment impediments, such as being brother and sister, or being married to another person at the time of the wedding, prevent such a marriage from being contracted at all. Such unions are called putative marriages.

Diriment impediments include:

• Consanguinity
• Insanity precluding ability to consent
• Not intending, when marrying, to remain faithful to the spouse (simulation of consent)
• One partner had been deceived by the other in order to obtain consent, and if the partner had been aware of the truth, would not have consented to marry
• Abduction of the woman, with the intent to compel her to marry (known as raptus), constitutes an impediment as long as she remains in the kidnapper's power. (In theory, the abduction of a man also constitutes an impediment, but no man has applied for annulment on these grounds.)
• Failure to adhere to requirements of canon law for marriages, such as clandestinity
• the couple killed the spouse of one of them in order to be free to marry
• the couple committed adultery, and one of the couple killed the spouse of one of them, in order to be free to marry

Some impediments can be dispensed, in which the church exempts a couple, prior to the marriage, to the obligation to conform to the canon law. While some relationships cannot have the impediment of consanguity dispensed, a marriage can be sanctioned between cousins. This renders the marriage non-annulable. Again, if an invalid marriage has been contracted, and the diriment impediment can be removed, a convalidation or sanatio in radice can be performed to make the marriage valid.

Marriages that are annulled under the Catholic Church are usually considered as ab initio, meaning that the marriage has been essentially invalid from the beginning. Some Catholics therefore worry that their children will be considered illegitimate if they get an annulment. However, Canon 1137 of the Code of Canon Law specifically affirms the legitimacy of children born in both recognized and putative marriages (those later declared null). Critics point to this as additional evidence that a Catholic annulment is similar to divorce—although civil laws that recognized both annulments and divorce regard the offspring of a putative marriage as legitimate.

An annulment verified by the Catholic Church is independent from obtaining a civil divorce, although before beginning a process in front of the Ecclesiastical Tribunal, it has to be clear that the marriage community cannot be rebuilt.

If someone has all the signs of being married previously, he or she must get an annulment before entering into a marriage in the Catholic Church, even if the individual was not married in the Catholic Church previously. Catholics acknowledge the indissolubility of marriage for any baptized persons who give themselves freely in the bond of marriage and recognizes the marriages of other Christians in most cases. However, it may decide not to recognize previous marriages involving Catholics conducted contrary to the Ne Temere requirements.