In Illinois, the circuit court is the court of original jurisdiction. There are 24 judicial circuits in the state, of which six are single county circuits (Cook, DuPage, Kane, Lake, McHenry and Will). The circuit court shares jurisdiction with the Supreme Court to hear cases relating to revenue, mandamus, prohibition, and habeas corpus. If the Supreme Court chooses to exercise its authority in a case of these types, the circuit court loses jurisdiction. The circuit court is also the reviewing court for certain state agency administrative orders.

There are two types of judges in the circuit court: circuit judges and associate judges.

Circuit judges are elected for a six-year term and may be retained by voters for additional six-year terms. They can hear any circuit court case. Circuit judges are initially elected either circuit-wide, from the county where they reside or from a sub-circuit within a circuit, depending on the type of vacancy they are filling. Circuit judges in a circuit elect one of their members to serve as chief circuit court judge. The chief circuit judge has general administrative authority in the circuit, subject to the overall administrative authority of the Supreme Court. The chief judge can assign cases to general or specialized divisions within the circuit.

Associate judges are appointed by circuit judges of that circuit, pursuant to Supreme Court Rule 39, for four-year terms. Associate Judges may not preside over criminal cases in which the defendant is charged with an offense punishable by imprisonment for one year or more (felonies), unless approval is received from the Supreme Court. Associate Judges are appointed by the Circuit Judges in accordance with Supreme Court rules for a four-year term.