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 California, an annulment (also known as a 'nullity of marriage') is a legal action where a court declares a marriage invalid from the start. Grounds for annulment in California include: (1) either party being underage without proper consent; (2) either party being of unsound mind or unable to understand the nature of the marriage; (3) fraud, which involves deceiving one party about something vital to the marriage; (4) force, if one party was compelled to marry against their will; (5) physical incapacity to consummate the marriage, which must have been unknown to the other party at the time of marriage; and (6) either party having a living spouse at the time of marriage. Additionally, if the marriage was entered into under the influence of alcohol or drugs, it can be annulled if the parties did not voluntarily cohabit after the effects of the substances wore off. The annulment must be sought within specific time frames, depending on the grounds. For instance, an annulment based on age must be filed before the underage person turns 18. If an annulment is granted, the marriage is considered void, as if it never occurred. These grounds are codified in the California Family Code, which provides the legal basis for annulments in the state.