A clear illustration of an affirmative defense is self-defense. In its simplest form, a criminal defendant may be exonerated, if he can demonstrate that he had an honest and reasonable belief that his conduct was necessary to protect himself or a third person against another's use of unlawful force.
Mistake of fact is another affirmative defense, usually used in combination with another, in which the defendant asserts that he reasonably believed the culpable act was necessary based on his observation, though given complete knowledge of the situation it is clear that the act was unwarranted. A person who assaults another person in defense of a third person that the victim had been attacking would be justified even if the victim and the third person were actors or performers, if the defendant could not reasonably have known this fact by observation.
Among the most controversial affirmative defenses is the insanity defense, whereby a criminal defendant, shown to be insane at the time of their crime, seeks commitment to a mental institution in lieu of imprisonment.
Most affirmative defense must be timely plead by a defendant in order for the court to consider them, or else they are considered waived by the defendant's failure to assert them. The classic unwaivable affirmative defenses is lack of subject-matter jurisdiction. What constitutes timely assertion is often itself the subject of contentious litigation.
Because an affirmative defense requires an assertion of facts beyond those claimed by the plaintiff, generally the party making an affirmative defense bears the burden of proof. The burden of proof is typically lower than beyond a reasonable doubt. It can either be proof by clear and convincing evidence or a preponderance of the evidence. In some cases or jurisdictions, however, the defense must only be asserted, and the prosecution has the burden of proving beyond a reasonable doubt that the defense is not applicable.
Rule 8 of the Federal Rules of Civil Procedure governs the assertion of affirmative defenses in civil cases filed in the United States district courts. Rule 8(c) specifically enumerates the following defenses: "accord and satisfaction, arbitration and award, assumption of risk, contributory negligence, discharge in bankruptcy, duress, estoppel, failure of consideration, fraud, illegality, injury by fellow servant, laches, license, payment, release, res judicata, statute of frauds, statute of limitations, waiver, and any other matter constituting an avoidance or affirmative defense."