Heat of passion or sudden passion is any intense, emotional excitement that prompts violent and aggressive action, such as rage, anger, hatred, furious resentment, fright, or terror that would cause an ordinary person to act on impulse and without reflection. When a crime such as murder is committed in the heat of passion and in response to a provocation—as opposed to being premeditated or deliberated—the provocation may serve as a partial defense to the crime by eliminating the premeditation element of a murder charge (also known as malice or malice aforethought), reducing the charge to manslaughter, for example.

The classic example of heat of passion provocation is when a spouse or lover finds his or her partner in an intimate place or sexual act with another person and shoots, stabs, or otherwise kills or attempts to kill one or both of them. The heat of passion defense is only available when the defendant reacts immediately to the provocation and without cooling off. If, for example, the defendant leaves and returns 30 minutes later to kill the cheating spouse, the heat of passion defense may not be available.

And in some states the heat of passion defense may also apply when the defendant had the honest but unreasonable belief he was confronted with deadly force, and used deadly force to defend himself.

Laws regarding the availability of the heat of passion defense vary from state to state. Some state laws downgrade a murder charge to manslaughter when the crime was committed under the influence of extreme mental or emotional disturbance for which there is reasonable explanation or excuse. These laws are generally located in a state’s statutes—often in the penal or criminal code.

And even when a partial defense of heat of passion or extreme mental or emotional disturbance may be available to a defendant, a conviction for manslaughter can carry a lengthy prison term and other serious consequences.

In Montana, the 'heat of passion' defense is recognized as a mitigating factor that can reduce a murder charge to manslaughter. This defense applies when a person commits a homicide under the influence of a sudden, intense emotion that would cause an ordinary person to act impulsively, without reflection. The provocation must be sufficient to incite an ordinary person to lose self-control, and the defendant must have acted immediately, without a 'cooling off' period. If there is a significant delay between the provocation and the act of killing, the defense is less likely to be successful. Montana law considers whether there was a reasonable explanation or excuse for the defendant's extreme mental or emotional disturbance at the time of the crime. However, even if a defendant successfully argues the heat of passion defense and the charge is reduced to manslaughter, the consequences can still be severe, including a lengthy prison sentence.