They go where other soldiers fear to tread, scoping out potential threats, taking out strategic targets and conducting daring rescue missions. These really are the best of the best.
Although it's difficult to rank these forces relative to one another, there are some units that rise above the rest in their track record and the fear they instill in their adversaries.
These soldiers have been through rigorous training exercises designed to weed out those who can't hit their exacting standards.
In a world where the importance of the sheer size of a country's military forces is no longer a guide to their effectiveness, these soldiers are the ones states look to in order to get the job done.
9. The Special Services Group in Pakistan is better known in the country as the "Black Storks" because of the commandos' unique headgear. Training reportedly includes a 36-mile march in 12 hours and a 5-mile run in 50 minutes in full gear.
In October 2009, SSG commandos stormed an office building and rescued 39 people taken hostage by suspected Taliban militants after an attack on the army's headquarters.
8. Spain's Unidad de Operaciones Especiales, or the Naval Special Warfare Force, as it has become known as since 2009, has long been one of Europe's best-respected special forces. Originally established as the volunteer Amphibious Climbing Company unit in 1952, it has since followed the British Special Air Service's example to become an elite fighting force.
Earning the UOE green beret, however, is a big ask with the failure rate of candidates averaging between 70% and 80%. It's not uncommon for 100% of would-be new recruits to be rejected.
7. Russia's Alpha Group is one of the best-known special forces units in the world. This elite antiterrorism unit was created by the KGB in 1974 and remains under its modern-day counterpart the FSB.
Russian special forces, and the Alpha Group in particular, came under criticism during the 2002 Moscow hostage crisis, in which 129 hostages died from the effects of the gas used to knock out militants who had seized a theatre.
6. Of all the counterterrorism forces in the world, few can compete with France's National Gendarmerie Intervention Group. The group is 200 strong and trained specifically to respond to hostage situations. They claim to have freed over 600 people since they were formed in 1973. It is against French law to publish pictures of their faces.
One of the most extraordinary episodes in the GIGN's history was the seizure of the Grand Mosque in Mecca in 1979. Because of the prohibition on non-Muslims entering the holy city, a team of three GIGN commandos briefly converted to Islam before helping the Saudi armed forces to plan the recapture of the mosque.
5. Israel's Sayeret Matkal is another of the world's most elite units. Its primary purpose is intelligence gathering, and it often operates deep behind enemy lines. During the selection camp (Gibbush), would-be recruits endure hardcore training exercises while being constantly monitored by doctors and psychologists. Only the strongest get in.
In 2003, Israeli taxi driver Eliyahu Gurel was kidnapped after transporting four Palestinians to Jerusalem in his cab. But the Sayeret Matkal unit located and rescued him from a 10-meter-deep pit in an abandoned factory in a suburb of Ramallah.
4. The UK equivalent of the US Navy's Sea, Air and Land (SEAL) teams is the Special Boat Service. The selection process involves a grueling endurance test, jungle training in the rain forests of Belize, and combat survival training, which involves intense interrogation of candidates. And you get only two attempts to pass.
3. To join the US Navy SEALs, you have to be able to do a minimum of 42
push-ups in two minutes, 50 sit-ups in two minutes, and run 1.5 miles in
11 minutes. And that's before training starts.
2. The US Army's Delta Force is even more elite that the SEALs. The counter-terrorist unit, formed in the mid-1970s, is highly secretive and tough on recruits. Former Delta operator Paul Howe said only 12 or 14 recruits qualified out of 240 men he tested.
1. The British SAS are the infantry counterparts to the Special Boat Service, dating back to 1941. The unit is often thought of as the "original" special forces unit and has inspired similar units around the world. Their insignia bears the famous phrase "Who dares wins." Asked about the importance of the SAS's role in the fighting that followed the Iraq war, US Gen. Stanley McChrystal responded, "Essential. Could not have done it without them."
BONUS: The US Marines are hardcore in their own right. Below, a US Marine drinks the blood of a cobra during a jungle survival exercise with the Thai Navy as part of the "Cobra Gold 2014" joint military exercise.