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 Maryland, an annulment is a legal decree that a marriage is null and void, meaning it is as if the marriage never legally existed. Grounds for annulment in Maryland include: lack of consent due to age (without parental consent or a court order if under 18), incapacity to consent due to drugs or alcohol, permanent impotence unknown to the other party at the time of marriage, fraud, duress, or force used to obtain consent, lack of mental capacity due to mental disorder, and marrying someone who was recently divorced where the petitioner was unaware of the divorce. Additionally, if a marriage ceremony occurred during a waiting period after the issuance of a marriage license, this could also be a ground for annulment. It is important to note that cohabitation after the discovery of the facts that provide a basis for annulment may affect the ability to obtain an annulment. These grounds are typically codified in the state's family law statutes. Individuals seeking an annulment should consult with an attorney to understand their rights and the legal process involved.