Hell Week is the defining event of BUD/S training for a Navy SEAL. It is held early on – in the 3rd week of First Phase – before the Navy makes an expensive investment in SEAL operational training. Hell Week consists of 5 1/2 days of cold, wet, brutally difficult operational training on fewer than four hours of sleep. Hell Week tests physical endurance, mental toughness, pain and cold tolerance, teamwork, attitude, and your ability to perform work under high physical and mental stress, and sleep deprivation. Above all, it tests determination and desire. On average, only 25% of SEAL candidates make it through Hell Week, the toughest training in the U.S. Military.