An answer must state affirmative defences, and the failure to plead one generally waives it.
The enumerated list in most procedural rules includes accord and satisfaction, arbitration and award, assumption of risk, contributory negligence, duress, estoppel, failure of consideration, fraud, illegality, laches, licence, payment, release, res judicata, statute of frauds, statute of limitations, and waiver. The list is not exhaustive.
Distinguish denials. A defence asserting that the plaintiff cannot prove an element is a denial, not an affirmative defence, and pleading it as the latter does no harm but does not shift any burden.
Specificity. Boilerplate lists of thirty defences without supporting facts are increasingly stricken, with courts applying plausibility principles to defences. Plead the facts supporting each defence that is actually available.
Late assertion. Courts often permit an unpleaded defence where it is raised without prejudice to the plaintiff, but a defence raised first at trial or on summary judgment is at the court’s discretion.
Jurisdictional defences — personal jurisdiction, venue, insufficient process and service — must be raised in the first responsive filing or are waived, which is the strictest of these rules.
Counterclaims. Compulsory counterclaims arising from the same transaction must be pleaded or are barred in later actions.