Short answer
An affirmative defense must be stated in the answer, or in a reply when the defense is responding to new matter, and the pleading must plainly set out the facts that make up the defense. The requirement covers named defenses like payment, release, fraud, duress, estoppel, laches, statute of frauds, illegality, contributory fault, void or voidable instruments, and failure of consideration, as well as any other affirmative matter that avoids or defeats the claim.1
Notice requirement
The statute also requires express pleading of any ground or defense — affirmative or not — that would likely take the other side by surprise if it were not stated.1
Illinois pleading rules reinforce that notice function: a pleading is not defective in substance if it reasonably informs the opposing party of the nature of the defense.2
Form and content
Defenses should be separately designated and numbered. A party may plead multiple defenses, including inconsistent defenses, and may state facts in the alternative or hypothetically when unsure which facts are true.1
All pleadings must use a plain and concise statement, and each defense must be separately pleaded, designated, numbered, and divided into consecutively numbered paragraphs.3
If a defense is founded on a written instrument, the relevant copy must be attached or recited in the pleading unless the pleader files an affidavit showing the instrument is not accessible.4
Challenging or fixing defects
If an affirmative defense is too vague or insufficient, the opposing party may seek a fuller or more particular statement, and objections to pleadings must be raised by motion that specifically identifies the defect.25
Before final judgment, the court may allow amendments on just and reasonable terms, including adding or changing defenses.6