An annulment is a lawsuit to have a court declare a marriage was invalid and the parties were never in fact married.

The grounds for an annulment vary from state to state but typically include: (1) the person seeking the annulment (the petitioner) was under age (18 years, for example) and did not have parental consent or a court order; (2) the person seeking the annulment (the petitioner) was under the influence of drugs or alcohol and did not have the capacity to consent to the marriage; (3) either party, for physical or mental reasons, was permanently impotent at the time of the marriage and the other party was not aware of the impotency; (4) the other party used fraud, duress, or force to induce the petitioner to enter into the marriage; (5) at the time of the marriage the petitioner did not have the mental capacity to consent to marriage or to understand the nature of the marriage ceremony because of a mental disease or defect; (6) the other party was divorced from a third party within the 30-day period preceding the date of the marriage ceremony, and at the time of the marriage ceremony the petitioner did not know, and a reasonably prudent person would not have known, of the divorce; and (7) the marriage ceremony took place before any waiting period (72 hours, for example) following issuance of the marriage license.

An annulment on any ground is often available only if the parties did not live together (cohabit) after the party seeking the annulment was no longer under the influence of drugs or alcohol, or learned of the facts that are the basis for the annulment sought. A marriage subject to annulment is often said to be a nullity, void, or void ab initio (void from the beginning).

The grounds for an annulment are usually found in a state’s statutes—often in the family code.

In New York, an annulment is a legal decree that a marriage is null and void—essentially, it is a declaration that a marriage never legally existed. Grounds for annulment in New York include: one party being underage without proper consent; lack of capacity to consent due to mental illness or intoxication; incurable impotence unknown to the other party at the time of marriage; consent to marriage obtained by force, duress, or fraud; or either party having a mental incapacity that prevents them from understanding the nature of the marriage contract. Additionally, if one party was still legally married to someone else at the time of the marriage, the new marriage is void. Unlike divorce, an annulment treats the marriage as though it never happened. However, for an annulment to be granted, the couple must not have cohabited after the issue that is the basis for the annulment became known. The specific statutes detailing annulment can be found in New York's Domestic Relations Law.