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 Washington State, an annulment is referred to as a 'Declaration Concerning Validity' and is governed by RCW 26.09.040. The grounds for an annulment in Washington are similar to those listed in the description. These include: one party being underage without requisite consent or court order; lack of capacity to consent due to intoxication or mental disability; impotency that was unknown to the other party at the time of marriage; fraud, duress, or force used to obtain the marriage; and a previous marriage that was not properly dissolved within 30 days prior to the current marriage. Additionally, Washington law requires that the parties have not cohabited after the condition that would make the marriage voidable ceased to exist or was discovered. If a marriage is annulled, it is treated as though it never legally existed, and the parties return to their single status. It is important to note that annulments are different from divorces, which dissolve a legally valid marriage, and are generally only granted under specific circumstances where the validity of the marriage itself is in question.