An affirmative defense exonerates a defendant of a criminal charge. Even if the defendant could otherwise be found guilty, a successful affirmative defense results in an acquittal. Among the most familiar affirmative defenses are "insanity" and "self-defense."

Because affirmative defenses require the consideration of facts and arguments beyond those asserted by the prosecution, the defendant generally has to prove the defense. The burden of proof is typically lower than beyond a reasonable doubt. It can either be "clear and convincing evidence" or "preponderance of the evidence".

Several APIS policy topics include information about affirmative defenses. For example, in Furnishing Alcohol to Minors, some States allow an affirmative defense for sellers/licensees when the minor has not been charged with an offense. In these States, an accused seller or licensee can defend by establishing that the minor was not charged. This type of affirmative defense arises out of legal concepts related to "fairness" or equity.

A more complex example is provided by the APIS policy topic Hosting Underage Drinking Parties: Criminal Liability.These laws hold individuals (social hosts) criminally responsible for underage drinking events on property they own, lease, or otherwise control. Insome jurisdictions, preventive action by the social host is an affirmative defense. To be acquitted, the defendant must prove that he or she took preventive action. However, in other jurisdictions, absence of preventive action is an element of the crime. In those jurisdictions, the prosecution must prove the social host did not take preventive action. That requirement makes prosecution more difficult.

One other APIS policy topic that includes an affirmative defense is False Identification for Obtaining Alcohol, Some States provide an affirmative defense to retailers who sell alcohol to underage purchasers based on a mistaken assessment of the purchaser's age. Other provisions similar to affirmative defenses affect the sanctions imposed on an offender rather than determination of guilt or innocence. The Beverage Service Training and Related Practices policy topic includes two such provisions. These provisions provide for mitigation of fines for sales to minors or sales to intoxicated persons, and protection against license revocation for sales to minors or sales to intoxicated persons. Statutory language sometimes refers to such provisions as "affirmative defenses." However, they differ from the affirmative defenses discussed earlier in that they do not apply to the evidentiary phase of court proceedings. Rather, they apply only after a judgment against the defendant has been reached.