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 Illinois, an annulment is legally referred to as a 'Declaration of Invalidity of Marriage.' The grounds for an annulment in Illinois are specified under the Illinois Marriage and Dissolution of Marriage Act. The reasons include: (1) one party was underage (under 18) and did not have parental consent or judicial approval; (2) either party was under the influence of drugs or alcohol and could not consent; (3) one party was unable to consummate the marriage due to a physical issue, and the other party was unaware at the time of marriage; (4) the marriage was entered into by force, fraud, or duress; (5) one party lacked the mental capacity to consent to the marriage; (6) one party was still legally married to someone else at the time of the marriage; and (7) the marriage license was obtained less than 24 hours prior to the ceremony. It is important to note that cohabitation after the discovery of the condition that provides the basis for the annulment can potentially invalidate the claim for annulment. An annulled marriage is considered void or non-existent from the beginning, as if it never legally occurred.