In Illinois, an annulment is referred to as a Declaration of Invalidity of Marriage. The court makes it as if the parties were never married. This can be important for a variety of reasons. For religious purposes, check with your clergy about what you need. The court can give you a civil annulment and your marriage will be wiped off the State’s books. As far as the government is concerned, it will be as if you were never married. Religious records can also be cleaned, but the court can’t do that for you. Talk with your clergy.
There are only a few situations that warrant a Declaration of Invalidity and they each have a fairly short time limit after the marriage ceremony within which a case must be filed. If you let the deadline pass, you give up your right to a Declaration of Invalidity and you'll have to stay married or seek a legal separation or a divorce.
I get a lot of calls from folks who want an annulment. Almost none of them qualify because the requirements are so difficult to satisfy. In fact, in over twenty years of practice, I've overseen the conclusion of only three annulments -- they almost never happen -- compared to somethign like 5,000 divorces. If you’re thinking about an annulment, you really probably need to start thinking about a divorce.
The grounds declaring a marriage invalid specified under Illinois law (and the time limits by which a case must be filed) are:
Coercion or duress: Within 90 days of learning of the condition. This refers to the classic "shotgun wedding." It's not coercion if your girlfriend says "marry me or I'll start dataing someone else." The case we learn in law school involved an Arab Sheik who took a young woman on his yacht and, 20 miles off shore, told her she could either marry him or swim back. That's coercion! Duress has been defined as " . . . the imposition, oppression, undue influence or the taking of undue advantage of the stress of another whereby one is deprived of the exercise of his own free will. . . . The person asserting duress has the burden of proving, by clear and convincing evidence, that he was bereft of the quality of mind essential to the making of the contract." In re: Marriage of Hamm-Smith, 261 Ill. App. 3d 209, 633 N.E.2d 225 (4th Dist., 1994)
Mental incapacity: Within 90 days of learning of the condition. This ground allows an annulment when one spouse didn't have the capacity to enter into the marriage contract either due to mental deficiencies or the influence of alcohol or drugs. It's the "I-was-so-drunk-I-didn't-know-what-I-was-doing-and-I-woke-up-with-this-ring-on-my-finger" excuse. Dennis Rodman used this excuse to annul his 9-day, Las Vegas marriage to Carmen Electra, and Britney Spears used it to annul her marriage to childhood friend Jason Alexander (not that Jason Alexander). The law varies by state but the law used in each cases is similar to that of Illinois.
Fraud -- Within 90 days of learning of the condition: Under Illinois' annulment law, the term "fraud" refers to a fraud involving "the essentials of the marriage." That generally means the ability to have kids -- it has nothing to do with representations as to wealth or social standing. If you thought you were marrying a millionaire but really you married a bum; congratulations, you're married to a bum and there's no way out but divorce. Physical incapacity to consummate the marriage -- Within one year of learning of the condition: This means sex; and the condition must have existed at the time of the marriage and the other spouse must not have been aware of this condition. One party was 16 or 17 -- Prior to the minor's 18th birthday: You can get around this if the under-age party has the consent of his or her parents (or guardians) or judicial approval. The marriage is otherwise prohibited -- At any time or, by a child of either party within 3 years of the death of the first party to die. This refers to parents marrying children, aunts marrying nephews, same-sex marriages (Illinois law still bans same=sex marriage as of the date of this publication -- 3 December 2012), etc. That's it. If you don't fall into one of these categories an dwithin the time limit, you're married and the only ways out are divorce or death.