A defense is asserted by a defending party in a pleading, typically an answer, to counter a claim made by another party. Defenses can be categorized into negative and affirmative defenses.

• A negative defense is a specific denial of material facts alleged by the claimant that are essential to their cause of action. Its purpose is to specifically deny the material averments in the opponent's pleading. A denial must be specific; a general denial is considered an admission. This requires the opposing party (e.g., the prosecution in a criminal case) to prove their case. Examples include denying specific factual allegations or stating lack of knowledge sufficient to form a belief as to the truth of a material averment, which has the effect of a denial. In criminal cases, denial and alibi are defenses that traverse allegations.

• An affirmative defense alleges a new matter which, while hypothetically admitting the material allegations of the claimant's pleading, would prevent or bar recovery. It functions as an "avoidance" of the claim. Affirmative defenses can also be grounds for dismissal of a complaint. Common examples include fraud, statute of limitations (prescription), release, payment, illegality, statute of frauds, estoppel, former recovery (res judicata), and discharge in bankruptcy. Specific grounds for dismissal that may be raised as affirmative defenses include lack of jurisdiction over the person or subject matter, improper venue, lack of legal capacity to sue, failure to state a cause of action, and failure to comply with a condition precedent. In criminal cases, affirmative defenses can include self-defense, and may modify the order of trial, requiring the accused to prove them.

Defenses are generally raised in the answer. Failure to raise defenses, particularly affirmative defenses, at the earliest opportunity (usually in the answer) can result in their waiver. However, certain defenses like lack of jurisdiction over the subject matter, litis pendentia, res judicata, and prescription are non-waivable. Affirmative defenses based on specific grounds for dismissal (like failure to state a cause of action or lack of jurisdiction over the person) may be resolved motu proprio by the court within a specified period after the answer is filed. If affirmative defenses are denied, they generally cannot be immediately challenged via motion for reconsideration or extraordinary writs but can be raised on appeal after a judgment on the merits. In certain special civil actions like expropriation or environmental cases, defenses must be raised in the answer or return and are subject to waiver if not.