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Criminal law
Part of the common law series
Elements of crimes
Actus reus  · Causation  · Concurrence
Mens rea  · Intention (general)
Intention in English law  · Recklessness
Willful blindness  · Criminal negligence
Ignorantia juris non excusat
Vicarious liability  · Corporate liability
Strict liability
Classes of crimes
Felony/Indictable  · Hybrid offence
Misdemeanor/Summary
Infraction
Lesser included offense
Crimes against the person
Assault  · Battery  · Robbery
Kidnapping  · Rape
Mayhem  · Manslaughter  · Murder
Crimes against property
Burglary  · Larceny  · Arson
Embezzlement  · False pretenses
Extortion  · Forgery  · Computer crime
Crimes against justice
Obstruction of justice  · Bribery
Perjury  · Misprision of felony
Inchoate offenses
Solicitation  · Attempt
Conspiracy  · Accessory
Subsets
Criminal procedure
Criminal defenses
Other areas of the common law
Contract law · Tort law  · Property law
Wills and trusts  · Evidence
Portals: Law  · Criminal justice

An affirmative defense is a category of defense used in litigation between private parties in common law jurisdictions, or, more familiarly, a type of defense raised in criminal law by the defendant. Affirmative defenses operate to limit or excuse or avoid a defendant's criminal culpability or civil liability, even if the factual allegations of plaintiff's claim are admitted or proven.

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 & reasonable belief that his conduct was necessary to protect himself against another's use of unlawful force.

Among the most controversial affirmative defenses is the insanity defense, whereby a criminal defendant, shown to be legally insane at the time of their crime, seeks commitment to a mental institution in lieu of imprisonment.

An affirmative defense must be timely pleaded by the defendant in order for the court to consider it, or else it is considered waived by the defendant's failure to assert it. What constitutes timely assertion is often itself the subject of contentious litigation.

Because affirmative defenses require the assertion of facts beyond those claimed by the plaintiff, generally the party making an affirmative defense has the burden of proof on that defense. 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.

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."

• civil law
  • assumption of risk (when the plaintiff knowingly entered into a dangerous situation)
  • authority
  • consent
  • defense of property
  • duress
  • estoppel
  • contract-specification defense
  • contractual provision (when the defendant's liability for causing the plaintiff's injuries had been waived in the contract)
  • contributory negligence (when the plaintiff's actions contributed to his own injury)
  • laches (similar to statute of limitations)
  • merger doctrine
  • repossession
  • statute of frauds
  • statute of limitations (too much time has elapsed between tort the and the bringing of the suit)
  • waiver
• criminal law
  • insanity defense
  • necessity
  • self-defense
  • statute of limitations