In military terminology, friendly fire or fratricide is an attack by belligerent or neutral forces on friendly troops while attempting to attack enemy or hostile targets. Examples include misidentifying the target as hostile, cross-fire while engaging an enemy, long range ranging errors or inaccuracy. Accidental fire not intended to attack enemy or hostile targets, and deliberate firing on one's own troops for disciplinary reasons is not called friendly fire, and neither is unintentional harm to civilian or neutral targets, which is sometimes referred to as collateral damage. Training accidents and bloodless incidents also do not qualify as friendly fire in terms of casualty reporting.

Use of the term friendly in a military context for allied personnel started during the First World War, often when shells fell short of the targeted enemy. The term friendly fire was originally adopted by the United States military; S.L.A. Marshall used the term in Men Against Fire in 1947. Many North Atlantic Treaty Organization (NATO) militaries refer to these incidents as blue on blue, which derives from military exercises where NATO forces were identified by blue pennants and units representing Warsaw Pact forces by red pennants. In classical forms of warfare where hand-to-hand combat dominated, death from a "friendly" was rare, but in industrialized warfare, deaths from friendly fire are more common.

Friendly fire is not to be confused with fragging, which is the uncondoned intentional (or attempted) killing of servicemen by fellow personnel serving on the same side.

Paul R. Syms argues that friendly fire is an ancient phenomenon. He notes recorded events in Ancient Greece and other early accounts of battles. He and other historians also note that weapons such as guns, artillery, and aircraft dramatically increased friendly-fire casualties.

By the 20th and 21st centuries, friendly-fire casualties have likely become a significant percentage of combat injuries and fatalities. Jon Krakauer provides an overview of American casualties during and since the Second World War:

While acknowledging that the "statistical dimensions of the friendly fire problem have yet to be defined; reliable data are simply not available in most cases," The Oxford Companion to American Military History estimates that between 2 percent and 25 percent of the casualties in America's wars are attributable to friendly fire.

In the annals of warfare, deaths at the hand of the enemy are often valorized, while those at the hand of friendly forces may be cast in shame. Moreover, because public relations and morale are important, especially in modern warfare, the military may be inclined to under-report incidents of friendly-fire, especially when in charge of both investigations and press releases:

If fratricide is an untoward but inevitable aspect of warfare, so, too, is the tendency by military commanders to sweep such tragedies under the rug. It's part of a larger pattern: the temptation among generals and politicians to control how the press portrays their military campaigns, which all too often leads them to misrepresent the truth in order to bolster public support for the war of the moment.