Divorce is dissolution of the marriage. It is a matter of state law.
Procedure. Divorces, whether contested or uncontested, are begun by filing a petition with an Illinois state court. In order to be eligible to receive a divorce in Illinois, at least one of the parties must be a resident of Illinois at the time the petition was filed, and at least one of the parties must have been a resident for at least 90 days before the divorce judgment is entered.
If the divorce is uncontested, which means that both parties agree to it, the process is fairly quick. The parties will draw up a divorce agreement setting out their rights as to their children, if any, and their property. Except in rare cases, the judge will grant the divorce and enter a judgment of divorce.
If the divorce is contested, the procedure is much like that of any civil suit, with one party filing the petition and the other party answering it. Evidence is gathered and a trial is held, unless the parties are able to reach an agreement prior to trial.
Grounds. Illinois recognizes both fault and no-fault divorce. The grounds for a fault divorce in Illinois are as follows:
1. adultery
2. attempted murder of the spouse
3. a felony conviction
4. bigamy
5. desertion (of one year or more)
6. extreme cruelty
7. habitual drunkenness or drug addition for at least two years
8. giving the spouse a sexually transmitted disease
9. permanent impotence at the time of marriage

The grounds for a no-fault divorce in Illinois are as follows:
1. the parties have lived separately continually for more than two years (two-year requirement can be waived if both parties want the divorce and they've lived apart for six months)
2. an irretrievable breakdown of the marriage has occurred due to irreconcilable differences
No-fault divorce laws exist to recognize the more modern fact that a party to a marriage should not be compelled to remain in the marriage simply because the other party hasn't been extremely cruel or committed a crime.
Annulments. Illinois recognizes annulments, which fall into one of two categories: void and voidable marriages. Void marriages are those that were illegal from the start, such as where the parties discover they are brother and sister. Voidable marriages are those where one party has the right to declare the marriage invalid, such as where one party falsely claimed that he or she was 18 at the time of marriage. A party who fails to act to declare the marriage void may be declared to have consented to the marriage and to have waived his or her right to terminate the marriage. The difference between an annulment and a divorce is that the law treats an annulled marriage as if it never took place.
Religious annulments are different from civic annulments. If, for example, someone were to have the Catholic Church annul her marriage, the legal status of the marriage would not be changed. Note also that children born of an annulled marriage are considered legitimate.
Legal separations. Illinois recognizes legal separations, in which the parties live apart but do not get divorced. Legal separations are used sometimes by those whose religious beliefs prohibit them from getting a divorce. In a typical legal separation document, the parties decide issues related to custody, child support, and alimony, but they do not decide issues related to their property. Any property, however, obtained after the agreement is entered is not considered part of the marital property should they later seek to dissolve the marriage. Couples subject to a legal separation agreement cannot remarry until the marriage is dissolved.