A sky marshal is a covert law enforcement or counter-terrorist agent on board a commercial aircraft to counter aircraft hijackings. Such an agent is also known as an air marshal, a flight marshal, a aviation safety officer (航空安全员) or an in-flight security officer (IFSO). Sky marshals may be provided by airlines such as El Al (who provide sky marshals on every flight), or by government agencies such as the Austrian Einsatzkommando Cobra, Royal Canadian Mounted Police, China Air Marshal Command [zh], German Federal Police, National Security Guard in India, Metropolitan Police MO19 (Specialist Firearms Command) from London, Pakistan Airports Security Force, or US Federal Air Marshal Service.

The history of in-flight security began in March 1971 when the US Federal Aviation Administration (FAA) directed a program to combat airplane hijackings. In that same year, there were numerous airplane hijackings in the United States that were planned with the ultimate aim to fly to Cuba. In response, the FAA created the title of FAA peace officer. FAA peace officers were the first people to provide armed security onboard commercial aircraft.

In response to the September 11 attacks, the commonwealth instituted an air security officer (ASO) program under the Australian Federal Police in December 2001. These officers are generally referred to in the media as "sky marshals". The ASO Program provides a discreet anti-hijacking capability for Australian civil aviation by putting armed security personnel on board aircraft. This involves both random and intelligence-led placement of armed ASOs on flights operated by Australian registered air carriers, on both domestic and international flights.

Officers are armed, trained and equipped for a variety of situations on both domestic and international flights.

In Austria, armed air marshals have been provided since 1981 by the Einsatzkommando Cobra.

The Canadian Air Carrier Protection/Protective Program (CACPP) began on September 17, 2002, when a memorandum of understanding was signed between the Royal Canadian Mounted Police with Transport Canada, the authority responsible for Canadian aviation security, and the Canadian Air Transport Security Authority (CATSA), for the implementation and administration of the CACPP. The program is conducted by specially trained undercover armed RCMP officers (known as "aircraft protective officers" – APOs) on selected domestic and international flights and all flights to Ronald Reagan Washington National Airport in the United States. Pilots and flight attendants are advised of their presence and the officer is authorized to physically intervene should an unauthorized person attempt to gain control of an aircraft. APOs, however, will not get involved in controlling unruly passengers. While they are peace officers within Canadian territories, they rely on section 6(2) of the Tokyo Convention as a legal basis for intervening in an incident outside of Canadian airspace. By law, such officers are exempt from acquiring a permit for importing or exporting their duty firearms when crossing the border. However, the exact nature of their weaponry is not released to the public except they are "deadly and effective and should not damage the aircraft". The Canadian Forces Military Police members of the Canadian Forces Air Marshal Detail, are responsible for providing security to Canadian forces aircraft, crew and passengers – passengers who may include the governor general, the prime minister and members of the royal family.

In 1973, China began having air marshals known as "Aviation Safety Officers" (航空安全员) on flights in 1973. In 1987, the State Council of China approved the establishment of private Aviation Safety Officer units by airlines.

In 2004, the China Air Marshal Command [zh] (中国民航空中警察总队) was established as an added name of the Civil Air Administration of China, with 1200 Aviation Safety Officers being sworn in as People's Police air marshals while 800 additional officers were transferred from local Public Security Bureaus.