Guerrilla warfare is a form of irregular warfare in which small groups of combatants, such as paramilitary personnel, armed civilians, or irregulars, use military tactics including ambushes, sabotage, raids, petty warfare, hit-and-run tactics, and mobility, to fight a larger and less-mobile traditional military.

Although the term "guerrilla warfare" was coined in the context of the Peninsular War in the 19th century, the tactical methods of guerrilla warfare have long been in use. In the 6th century BC, Sun Tzu proposed the use of guerrilla-style tactics in The Art of War. The 3rd century BC Roman general Quintus Fabius Maximus Verrucosus is also credited with inventing many of the tactics of guerrilla warfare through what is today called the Fabian strategy. Guerrilla warfare has been used by various factions throughout history and is particularly associated with revolutionary movements and popular resistance against invading or occupying armies.

Guerrilla tactics focus on avoiding head-on confrontations with enemy armies, typically due to inferior arms or forces, and instead engage in limited skirmishes with the goal of exhausting adversaries and forcing them to withdraw. Due to this, guerrilla tactics are rarely used for anything other than defence. Organized guerrilla groups often depend on the support of either the local population or foreign backers who sympathize with the guerrilla group's efforts.