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 Wisconsin, an annulment is a legal procedure to declare a marriage invalid, as if it never occurred. The grounds for annulment in Wisconsin include: 1) one party was underage (under 18) without parental consent or a court order; 2) incapacity to consent due to intoxication from drugs or alcohol; 3) incurable impotence unknown to the other party at the time of marriage; 4) the marriage was entered into because of fraud, duress, or force; 5) lack of mental capacity to consent due to mental disease or defect; 6) one party was recently divorced (within 30 days before the marriage) and the other party was unaware; and 7) the marriage occurred during a prohibited waiting period after the marriage license was issued. Annulment is typically only an option if the couple did not cohabit after the basis for annulment became known. A marriage that is annulled is considered void or 'void ab initio', meaning it is treated as never having existed. These grounds are codified in Wisconsin's statutes, specifically within the family code.