A sky marshal (also known as air marshal or flight marshal) is an undercover law enforcement or counter terrorist agent on board a commercial aircraft to counter aircraft hijackings. 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, German Federal Police, National Security Guard in India, Metropolitan Police SO18 (Aviation Security Operational Command Unit) or US Federal Air Marshal Service.
In response to the September 11 incidents, 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 Programme provides a discreet anti-hijacking capability for Australian civil aviation by providing 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.
In Austria, air marshals are 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 the Ronald Reagan Washington National Airport. Pilots and flight attendants are advised of their presence and the officer will physically intervene should an unauthorized person attempt to gain control of an aircraft. APOs, however, will not be 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 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 1 Canadian Air Division VIP Aircraft Security 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.
After the explosion on Air India Flight 182 and the hijacking of Indian Airlines Flight 814, Indian civil airline regulators decided to deploy sky marshals on all its international flights. Post the September 11 attacks, the Bureau of Civil Aviation Security ordered sky marshals on all domestic and international flights. Presently, all Air India flights carry sky marshals. Private operators like Jet Airways also carry Air Marshals on selective routes. The air marshals are recruited from India's elite commando force National Security Guard.
Sky Marshals are rarely used on flights originating from the UK. Officers from The Metropolitan Police SO18 Aviation Security branch are used on random flights, however details of the programme are not made public and the extent of their use is not widely known. For this reason their use is not heavily documented in the media.
The US Federal Aviation Administration began its "Sky Marshal" program in 1968, which eventually became the Federal Air Marshal Service in 1985. That year, an Egyptian security officer on board EgyptAir Flight 648 opened fire on three Abu Nidal Organization hijackers, killing one before being fatally shot himself. In 2005, Rigoberto Alpizar was shot dead by two sky marshals on a jetway at Miami International Airport. Currently, Federal Air Marshal Officers are under Transportation Security Administration. Under the Visible Intermodal Prevention Response (VIPR) system, started up circa 2005, FAMs began to patrol non-aviation sites like bus terminals and train stations.
On May 30, 2013, the first book ever written on the history of the air marshals was published in the United States by former air marshal Clay Biles. The book, titled The United States Federal Air Marshal Service: A Historical Perspective, 1962 - 2012 gave never before released information on the more than fifty years of Federal Air Marshal Service. The author interviewed a number of former Sky Marshals and contributed significantly to reversing the lack of information on this short-duration program. 
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In the film Harold & Kumar Escape from Guantanamo Bay, air marshals tackle the titular protagonists to the ground after a passenger sees Kumar's marijuana bong in the lavatory, believing it to be a bomb, and the smoke coming from it poison gas; they are then sent to Guantanamo Bay as suspected terrorists.
In the movie Get Smart, the main character Maxwell Smart is mistakenly tackled by a sky marshal while beginning a mission on board a plane.
Richard Tyson plays Federal Air Marshal Paul Judd of the TSA in the low-budget horror film Flight of the Living Dead.
In the film Bridesmaids, one of the main characters, Megan, is seated next to 'Air Marshal John', who repeatedly denies being an air marshal until trouble commences.
In the novel Rainbow Six by Tom Clancy the two main protagonists foil a hijacking attempt on their way from the United States to the United Kingdom; after this they then pose as having been Air Marshals so as not to blow their cover as a covert special forces team that just happened to be on the hijacked aircraft.
In the series NCIS a flight marshal is killed (Season 7, Episode 13)
|Look up sky marshal in Wiktionary, the free dictionary.|
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- See article on Visible Intermodal Prevention Response