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 Montana, an annulment is a legal procedure that declares a marriage invalid, as if it never occurred. The grounds for annulment in Montana include: lack of legal age without parental consent or a court order; incapacity due to intoxication by drugs or alcohol; permanent impotency unknown to the other party; fraud, duress, or force used to obtain marriage consent; mental incapacity to consent due to a mental disorder; ignorance of the other party's recent divorce within 30 days prior to the marriage; and non-compliance with any mandatory waiting period after obtaining a marriage license. It is important to note that cohabitation after the discovery of the grounds for annulment may affect the ability to seek an annulment. These grounds are typically codified in the state's statutes, which in Montana can be found under the family law section of the Montana Code Annotated (MCA). An attorney can provide specific guidance on the process and likelihood of obtaining an annulment based on individual circumstances.