In most litigation under the common law adversarial system the defendant, perhaps with the assistance of counsel, may allege or present defenses (or defences) in order to avoid liability, civil or criminal.

Criminal law defenses

In criminal law these defenses can be grouped into 4 categories:

• Alibis;
• Justifications;
• Excuses;
• Procedural defenses, and the so-called
• "Innovative defenses."

With the exception of alibis; these defenses do not argue that the defendant didn't commit a criminal act (which broke the law); but rather, that the defendant should not be held criminally liable for said act.

Civil law defenses

In civil law affirmative defenses are presented to limit or avoid liability, they are numerous and include:

• Pleading;
• The statute of limitations;
• Lack of personal or subject matter jurisdiction of the court, or
• Failure to state a cause of action.