Pegasus Company selection has been called one of the most difficult selection courses in the British Army.
The course is run by members of the Parachute Regiment at Catterick Garrison in North Yorkshire.
Over the three weeks of the course, a steady flow of people, hoping to become airbourne soldiers, give up and leave the course, starting from day one.
The BBC's Jonathan Swingler spoke to the soldiers as they tried to pass Pegasus Company selection.
Candidates hope to become part of Pegasus Company.
Glen Hill from Hartlepool is 35-years-old and wants to become an airborne soldier.
Making his way along a 10 mile run with a 16kg (35lb) rucksack he said, "All the guys here want to go to Afghanistan to help our mates out there".
A mile later, one of the soldiers was already in the back of a truck, having failed to keep up with the rest.
Yet this is seen as one of the easier tests. Later, they would have to run 20 miles carrying the same weight.
The most daunting challenge came a week later. The soldiers were split into teams and made to sprint across difficult terrain carrying a log. Glen Hill's team crossed the finish line first.
The exercise lasts less than 20 minutes, but the physical exertion required to complete it in that time was so severe that, by the end, 16 soldiers were standing by the side of the route waiting for the truck to pick them up.
Milling is a one minute exercise
The next day they found themselves in a sports hall, preparing to take part in "milling".
They wore gloves, but there the similarity with boxing ended. Participants in milling are not allowed to block or avoid incoming punches.
Major Mark Swan gave a briefing before the first bout. "Milling; the army medical services don't like it. Why do we do it?
"Some of you are going to go to Afghanistan fairly soon. Every ounce of your being will tell you to stay in cover; that's useless to us.
"We need men who are going to get in the fire position, look through their sights and engage the enemy."
Nathan Haswell, a 19-year-old from Tyne and Wear, easily wins his first bout. His opponent was being looked at by the medic, because blood was coming from his nose. After his second fight though, the teenager was struggling to catch his breath: "That was hard".
Dr Rob Zelenka, a civilian GP looking after the men here, said the decision to allow milling was far above his pay grade. "I am happy with the steps that have been put in place".
By the final test week, the staff in charge of the course had deliberately tired the men out.
The objective, they said, was to reflect how they might feel towards the end of a six month tour in Helmand.