Annulment is a term that has come to be used for the declaration by the Church of the nullity of a marriage. A decree of nullity is the Church's declaration that, after careful scrutiny and consideration by a judicial process, it has been found that the marriage had been invalid right from the beginning owing to a lack, on the part of one or both of the parties, of the qualities necessary for a valid marriage, inspite of appearances to he contrary, such as the externally correct celebration of the wedding.

In the early centuries of the Christian era, that is, up to about the tenth century, marriage problems were looked into and settled informally by priests, bishops and the Pope. By the end of the 10th century, however, judicial structures and procedures for marriage cases began to emerge. They developed in the succeeding centuries both at diocesan level and at the center of Church government in Rome. The Roman Rota eventually became the highest Church tribunal for marriage annulments. The 18th century saw the standardization of the practice relating to judicial procedures. The laws governing it were incorporated into the First Code of Canon Law in 1917. The New Code of 1983 brought in changes, especially for a more pastoral view of problems affecting marriage.

A petition for investigation by a tribunal is accepted only if there is prima facie evidence that a marriage has taken place and there is serious doubt whether or not the conditions for validity were fulfilled. The Code of Canon Law lists the impediments that make a marriage invalid, such impediments as lack of prescribed age, impotence, being married already or in sacred orders or bound by a vow of perpetual chastity, consanguinity and affinity or spiritual relationships. It is only with a dispensation from Church authority, where such dispensation is allowable, that a person subject to an impediment can be validly married (Cans. 1083-1094). Of great importance is the free consent of the contracting parties based on a correct knowledge of what marriage entails. The absence of such consent renders the marriage null and void.

The Church makes use of the new knowledge and insights gained in the social sciences, especially psychology, in handling nullity cases. The admissibility of psychological evidence is a new feature in the procedure of marriage tribunals.

One does not incur excommunication by divorce or remarriage. However, a person who is divorced and remarried civilly cannot receive the Eucharist, since the new relationship is tantamount to adultery.