In the criminal law, duress, also called coercion, may be used in trial to exculpate the defendant of guilt. Because the defendant did actually commit an actus reus and some measure of mens rea is present simply because the defendant did actually intend to commit the crime, some degree of liability is already attached. However, if the defendant commits a criminal offense while under severe duress, that is, restraint or the threat of real and tangible harm, then the mens rea of the act may be lessened or even eliminated altogether.

Duress can nullify liability for a criminal act if actual violence or the threat of violence compel a person to commit or omit an action in violation of the law. There are generally four instances that provide exculpation of a defendant's guilt by virtue of this defense. These are, for fear of loss of life, or dismemberment, or mayhem (loss of limb or sever disfigurement), or unlawful imprisonment. The defense of duress may apply when these acts of violence, or threats of violence, are inflicted not only on the defendant, but also on the defendant's spouse, children, or other immediate family members or dependents.

Annoyance and persuasion are usually not enough to sustain a defense under the terms of duress. Nor is just any degree of violence or threat usually enough. The duress defense generally requires that the violence or threat of violence completely overcame the courage of the defendant - as it would have overcome the courage of any person of normal courage - such that he lost control of his will power and essentially acted involuntarily and is thus not liable for his actions. Many factors must be taken into account when determining what the defendant's normal level of courage should have been, including his or her age, weight, height, health, temper and disposition.