Special Forces selection includes a series of five timed marches across the Brecon Beacons in Wales. As well as testing a soldier's navigational skills, this challenges his physical and mental strength to the limit.
'It's the equivalent of doing six marathons over five days, over the mountains, with weight on your back, with nobody to help or push you.'
Robin Horsefall, Special Air Service veteran
Special Forces soldiers have to survive and fight in extreme environments. Recruits must therefore pass a jungle survival course that tests their determination, endurance and adaptability.
'Everything in the jungle wants to kill you. Everything. Even the plants!'
Jason Fox, Special Boat Service veteran
Armies use parachutes as a means of quickly delivering men and equipment into a battle zone. This might involve a mass drop of thousands of paratroopers, or a handful of Special Forces soldiers being secretly dropped into enemy territory.
'So, you're basically involved in a car crash. I don't know anybody who says, "I think I'll have a bit of a car crash today... or a couple."'
Tim Collins, Special Air Service veteran