Elite special forces are some of the best-trained and most formidable units a country can boast.
They go where other troops fear to tread, scoping out potential threats, taking out strategic targets, and conducting daring rescue missions.
These really are the best of the best.
Though it's extremely 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 troops have been through rigorous training exercises designed to weed out those who can't hit their exacting standards.
In a world in which the sheer size of a country's military is no longer the only guide to its effectiveness, these troops are the ones that states look to in order to get the job done.
Note that there are some graphic images below that some readers may find disturbing.
Tomas Hirst contributed to a previous version of this post.
8. The Special Services Group, SSG, 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.
7. Spain's Unidad de Operaciones Especiales, or the Naval Special Warfare Force as it has become 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 SAS' example to become an elite fighting force.
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