People are often confused by the rule that an appellate court reviews errors of law, not errors of fact.  What does that mean? 

Think about baseball.  If the umpire calls a batter out on two strikes then the batter's team may be able to protest the call and have the game replayed, because the rules allow the batter three strikes, not two.  So the umpire got the rule wrong.

But if the umpire thinks that the ball is over the plate and everybody else thinks that it isn't, the umpire's call stands, because the  mistake is about the facts, not the rules, and the umpire's call on the facts is final.

An appeal works the same way.  If the trial judge, for example, allows inadmissible hearsay into evidence, the appellate court may order a new trial because the judge made a mistake of law; that is, the judge got the rule wrong.

But if the jury, or the judge at a bench trial, decides that a witness who has made conflicting statements is nevertheless believeable, the appellate court will not interfere.  The jury or the trial judge has the final say on all factual questions, including what witnesses to believe.

An appeal is a review of a judgment by a higher court.  In Illinois an appeal is normally taken to the appellate court, which has five geographic districts covering the entire state between them.  The appellate court sits in panels of three judges. These judge hear no witnesses and accept no new evidence.  Their review is confined to the record made in the trial court, and they review the record for legal error, in other words, for mistakes of law made by the trial judge that may have rendered the outcome of the trial unfair or unreliable.

Almost all of the work before a court on appeal is done in writing.  The judges have for consideration a record including all of the documents filed in the case in the trial court, transcripts of all proceedings that took place in court, all exhibits that were admitted into evidence and any refused exhibits as to which there may be an issue on appeal.  They also have case books, called briefs, prepared by the lawyers.  A typical brief in an Illinois criminal appeal might be anywhere from fifteen to fifty pages long.  The attorney for the defendant who is appealing a criminal conviction seeks to persuade the court that the trial  judge made mistakes of law so serious as to make the trial unfair or to have made the outcome of the trial unreliable.  The prosecutor's brief tries to convince the court that the judges rulings were correct or, if they were wrong, that they were too unimportant to matter.

The judges, after studying the briefs, reviewing the record, and hearing a short oral argument at which they often pepper the attorneys with technical questions and challenge their reasoning, will make their decision in writing.

An appeal is initiated by filing a notice of appeal with the clerk of the trial court.  The notice of appeal is normally due thirty days after final judgment.  The clerk of the trial court must prepare the record and transmit it to the appellate court.  The appellant, usually the defendant taking the appeal, files a brief first.  The appellee, usually the prosecutor, files a brief in response.  The appellant has the option to file a reply brief.  When all briefs are filed the court will either order the attorneys to appear for oral argument or will choose to decide the case on the briefs without hearing argument. 

The decision is made in writing, sometimes weeks, sometimes months, after briefing and any oral argument are completed.  The court can reverse a criminal conviction and order the defendant released, it can reverse and remand the case and order that it be retried, it can uphold the conviction but either reduce the sentence or return the case to the trial court for resentencing, or it can simply affirm, which means that it finds no significant error and upholds the judgement of the trial court.

A party who loses an appeal before the appellate court can seek further review from the Illinois Supreme Court by filing a petition for leave to appeal.  Such petitions are allowed in a distinct minority of cases, but in the appropriate case the Illinois Supreme Court does not hesitate to reverse the appellate court. 

If there are issues of federal law involved in an Illinois state court case, review from the denial of relief by the Illiois Supreme Court can be sought, as to those federal issues only, by filing a petition for writ of certiorari asking the United States Supreme Court to accept the case.  Such a petition, however, is seldom granted.