In military tactics, close air support (CAS) is defined as aerial warfare actions—often air-to-ground actions such as strafes or airstrikes—by military aircraft against hostile targets in close proximity to friendly forces. A form of fire support, CAS requires detailed integration of each air mission with fire and movement of all forces involved. CAS may be conducted using aerial bombs, glide bombs, missiles, rockets, autocannons, machine guns, and even directed-energy weapons such as lasers.

The requirement for detailed integration because of proximity, fires or movement is the determining factor. CAS may need to be conducted during shaping operations with special forces if the mission requires detailed integration with the fire and movement of those forces. A closely related subset of air interdiction, battlefield air interdiction, denotes interdiction against units with near-term effects on friendly units, but which does not require integration with friendly troop movements. CAS requires excellent coordination with ground forces, typically handled by specialists such as artillery observers, joint terminal attack controllers, and forward air controllers.

World War I was the first conflict to make extensive use of CAS, albeit using relatively primitive methods in contrast to later warfare, though it was made evident that proper coordination between aerial and ground forces via radio made attacks more effective. Several conflicts during the interwar period—including the Polish–Soviet War, the Spanish Civil War, the Iraqi Revolt, and the Chaco War—made notable use of CAS. World War II marked the universal acceptance of the integration of air power into combined arms warfare, with all of the war's major combatants having developed effective air-ground coordination techniques by the conflict's end. New techniques, such as the use of forward air control to guide CAS aircraft and identifying invasion stripes, also emerged at this time, being heavily shaped by the Italian Campaign and the invasion of Normandy. CAS continued to advance during the conflicts of the Cold War, especially the Korean War and the Vietnam War; major milestones included the introduction of attack helicopters, gunships, and dedicated CAS attack jets.