US Army Ranger School is considered the toughest combat course on the planet. The course replicates the nonstop stress of war to forge the Army's elite combat leaders.

For sixty-one days and nights, this extreme training replicates the nonstop stress of warfare. Soldiers sleep an average of three hours a night and receive just one meal per day. They train continuously, running missions 20 hours straight in mountains, swamps and dense woodlands. Most lose 20 or more pounds before it's over, and many quit or fail the course along the way. Only one in three make it through.

Air Force Pararescuemen are the most highly trained combat search and rescue experts in US Special Operations. When a pilot goes down behind enemy lines, these are the experts who stop at nothing to get him out. Nicknamed "PJ's" for, they are also surgically trained combat medics.

Full training to become a PJ takes more than two years. Early on, students must survive one notoriously difficult milestone called "Extended Training Day" designed to weed out anyone not up to the staggering demands of the PJ job. It's 24 hours of pure hell: nothing else in any special operations training course rivals the torment of this single day. In the end, only nine men out of 100 survive the cut and are allowed to continue training as a PJ.

Reconnaissance marines are the eyes and ears of the Marine Corps. The first men on the ground behind enemy lines, they gather intelligence and carry out missions that support the entire Corps. Their motto says it all: Swift, Silent, Deadly.

But to earn the name "Recon Marine", you must first survive the corps' famously grueling 12-week recon course. Marines are pushed to unconsciousness in the pool, and then wrestle their 90-pound packs into the pounding surf of the Pacific Ocean. It's a man-breaking, all-out endurance test that forges top soldiers with unparalleled skills.

Navy Explosive Ordnance Disposal technicians or EOD are the only explosives and bomb specialists qualified for special operations.

Before a Navy EOD team can go to war with Special Forces, they must go through one final test. Like the job itself, it's all-or-nothing training: A five-day, non-stop series of high stress missions. Make the wrong move, allow fatigue or distraction to take over and the entire team will fail. The team that survives the cut stays together as a certified special ops team.

Marine Snipers are legendary in the sniper world for being the best. This is where they learn their craft. Units will only send their best men to this course, but on average 1 in the 3 will fail. Becoming a marine sniper takes a level of discipline that few could ever hope to achieve. They will wait hours in the worst possible conditions but only have seconds to make the shot.