Common law marriage—also known as marriage without formalities or informal marriage—is marriage without a wedding or marriage ceremony and without a marriage license. Some states recognize common law marriage under certain circumstances—such as when the parties agree to be married, live together (cohabit) after agreeing to be married, and represent to others that they are married (holding themselves out as a married couple).

Other states previously recognized common law marriages, but no longer recognize such marriages after a certain date on which the law was changed—usually by statute. In other states, common law marriage has not been recognized by the legislature in a statute, but the courts have recognized common law marriage in court opinions or case law.

In Illinois, common law marriages are not recognized. This means that couples who cohabit and hold themselves out as married without formalizing their relationship through a marriage license and ceremony do not have the legal status of being married under Illinois law. Illinois law requires couples to obtain a marriage license and have a marriage ceremony to be legally recognized as married. However, Illinois does recognize common law marriages that were established in other states where such marriages are legally recognized, provided they meet the requirements set forth by the state where the marriage was established. This recognition is based on the principle of comity, which allows for the acknowledgment of legal statuses or judgments made under the laws of other states.