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 "guerilla warfare" was coined in the context of the Peninsular War in the 19th century, the tactical methods of guerrilla warfare have been in use since long before then. 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. The tactics of guerrilla warfare have been used by various different factions throughout history, and are particularly associated with revolutionary movements and popular resistance against invading or occupying armies.

Guerrilla tactics focus on avoiding head-on confrontations with enemy armies, instead engaging in limited skirmishes with the goal of exhausting their adversaries and eventually forcing them to withdraw. Guerilla groups often depend on the logistical and political support of either the local population or foreign backers who do not engage in armed struggle but sympathize with the guerrilla group's efforts.