Non-lethal weapons, also called nonlethal weapons, less-lethal weapons, less-than-lethal weapons, non-deadly weapons, compliance weapons, or pain-inducing weapons are weapons intended to be less likely to kill a living target than conventional weapons such as knives and firearms with metal-projectile ammunition. It is often understood that unintended or incidental casualties are risked wherever force is applied; however, non-lethal weapons minimise the risk of casualties (e.g. serious/permanent injuries or death) as much as possible. Non-lethal weapons are used in policing and combat situations to limit the escalation of conflict where employment of lethal force is prohibited or undesirable, where rules of engagement require minimum casualties, or where policy restricts the use of conventional force. However, these weapons occasionally cause serious injuries or death due to allergic reactions, improper use and/or other factors; for this reason the term "less-lethal" has been preferred by some organizations as it describes the risks of death more accurately than the term "non-lethal", which some have argued is a misnomer.

Non-lethal weapons may be used by conventional military in a range of missions across the force continuum. They may also be used by military police, by United Nations forces, and by occupation forces for peacekeeping and stability operations. Non-lethal weapons may also be used to channelize a battlefield, control the movement of civilian populations, or to limit civilian access to restricted areas (as they were utilized by the USMC's 1st Marine Expeditionary Force in Somalia in 1995). Similar weapons, tactics, techniques and procedures are employed by police forces domestically in riot control, prisoner control, crowd control, refugee control, and self-defense, where the terminology of "less-than-lethal" is often used.

In the past, military and police faced with undesirable escalation of conflict had few acceptable options. Military personnel guarding embassies often found themselves restricted to carrying unloaded weapons. National guards or policing forces charged with quelling riots were able to use only batons or similar club-like weapons, or bayonet or sword charges, or fire live ammunition at crowds. In the late 1980s and early 1990s, the Non-lethality Policy Review Group at U.S. Global Strategy Council in Washington and other independent think tanks around the world called for a concerted effort to develop weapons that were more life-conserving, environmentally friendly, and fiscally responsible than weapons available at that time. The U.S. Congress and other governments agreed and began an organized development of non-lethal weapons to provide a range of options between talking and shooting.

Recognizing the need to limit the escalation of force, research and development of a range of non-lethal weapons has since been undertaken internationally by governments and weapons manufacturers to fill the need for such weapons. Some non-lethal weapons may provide more effective riot control than firearms, truncheons or bayonets with less risk of loss of life or serious injury. Before the general availability of early military non-lethal weapons in the mid 1990s, war-fighters had few or no casualty-limiting options for the employment of scalable force and were continually at risk whenever lethal force was prohibited during sensitive missions.

In 2001, the United States Marine Corps revealed its development of a less-than-lethal energy weapon called the Active Denial System, a focused high frequency microwave device said to be capable of heating all living matter in the target area rapidly and continuously for the duration of the beam, causing transient intolerable pain but no lasting damage. The skin temperature of a person subjected to this weapon can jump to approximately 130 °F (54 °C) in as little as 2 seconds depending on the skin's starting temperature. The system is nonlethal (the penetration of the beam into human skin is only a few millimeters).

In 2004, author Jon Ronson cited an unclassified military report titled "Non-Lethal Weapons: Terms and References" 21 acoustic weapons were listed, in various stages of development, including the Infrasound ("Very low-frequency sound which can travel long distances and easily penetrate most buildings and vehicles ... biophysical effects are projected to be: nausea, loss of bowels, disorientation, vomiting, potential internal organ damage or death may occur. Superior to ultrasound...)", however no such effects had been achieved as of 2002^[update].

In 2010, the Joint Non-Lethal Weapons Directorate Non-Lethal Weapons Reference Book was created. The weapons in this book are currently in development.

Until the development of non-lethal weapons, police officers around the world had few if any non-lethal options for riot control. Common tactics used by police that were intended to be non-lethal or less lethal included a slowly advancing wall of men with batons, officers on horses trained to deal with policing situations, or a charge into a riot using the flats of sabers. Other reasonably successful approaches included shotguns with lower-powered cartridges, "salt shells", using bean-bag rounds and ricocheting shots off of the ground. In the mid-20th century, with the integration of fire-control systems into major cities, police found that high-pressure fire hoses could be effective in dispersing a crowd (the use of water cannons and fire trucks has remained an effective non-lethal tactic to disperse riots). Trained police dogs were also commonly used to scare and disperse rioters and apprehend individuals. In the 1980s the development of high-tensile plastics like Kevlar and Lexan revolutionized personal armor and shields, and led to new tactics for riot squads and other special-purpose teams. Officers could now stand up against violent rioters throwing dangerous projectiles without having to resort to lethal methods to quickly disperse the danger. Coupled with the introduction of effective non-lethal chemical agents such as tear gas and offensive-odor canisters, and non-lethal impact rounds such as rubber bullets and "bean bag" flexible baton rounds, riot tactics were modified to rely less on violent response to attacking rioters than on a return to the slowly advancing wall, with supporting officers firing non-lethal ordnance into the crowd to discourage advance.^{[citation needed]}