Deadly force, as defined by the United States Armed Forces, is the force which a person uses, causing—or that a person knows, or should know, would create a substantial risk of causing—death or serious bodily harm. In most jurisdictions, the use of deadly force is justified only under conditions of extreme necessity as a last resort, when all lesser means have failed or cannot reasonably be employed.

Firearms, bladed weapons, explosives, and vehicles are among those weapons the use of which is considered deadly force. The use of non-weapons in an aggressive manner, such as a baseball bat or tire iron, may also be considered deadly force.

"Use of deadly force" is often granted to police forces when the person or persons in question are believed to be an immediate danger to people around them. For example, an armed man in a shopping mall shooting at civilians without regard to the safety of anyone around him, and refusing or being unwilling to negotiate, would warrant usage of deadly force, as a means to prevent further danger to the community. The use of deadly force is also authorized when a person poses a significant threat to a law enforcement officer, usually when the officer is at risk of serious bodily injury or death. In the United States this is governed by Tennessee v. Garner, which said that "deadly force...may not be used unless necessary to prevent the escape and the officer has probable cause to believe that the suspect poses a significant threat of death or serious physical injury to the officer or others." In Australia, it has recently been proposed that police officers should have this power when a person might in the future pose a threat to others (see Australian Anti-Terrorism Act 2005).

Massad F. Ayoob (born 20 July 1948) is an internationally-known firearms and self-defense instructor. He has taught police techniques and civilian self-defense to both law enforcement officers and private citizens in numerous venues since 1974. He was the director of the Lethal Force Institute (LFI) in Concord, New Hampshire from 1981 to 2009. He now directs the Massad Ayoob Group (MAG). Ayoob has appeared as an expert witness in several trials. He has served as a part-time police officer in New Hampshire since 1972 and holds the rank of Captain in the Grantham, New Hampshire police department.

Ayoob has authored several books and more than 1,000 articles on firearms, combat techniques, self-defense, and legal issues, and has served in an editorial capacity for Guns Magazine, American Handgunner, Gun Week, Guns & Ammo and Combat Handguns. Since 1995, he has written self-defense and firearms related articles for Backwoods Home Magazine. He also has a featured segment on the television show Personal Defense TV, which airs on the Sportsman Channel in the US.