Waterboarding is a form of torture that consists of immobilizing a person on their back with the head inclined downward (the Trendelenburg position), and pouring water over the face and into the breathing passages. Through forced suffocation and inhalation of water, the subject experiences the process of drowning and is made to believe that death is imminent. In contrast to merely submerging the head face-forward, waterboarding almost immediately elicits the gag reflex. Although waterboarding does not always cause lasting physical damage, it carries the risks of extreme pain, damage to the lungs, brain damage caused by oxygen deprivation, injuries (including broken bones) due to struggling against restraints, and even death. In a letter to Attorney General Alberto Gonzales more than 100 United States law professors stated unequivocally that waterboarding is torture, and the use of the practice is a criminal felony punishable under the U.S. federal criminal code. The psychological effects on victims of waterboarding can last for years after the procedure.

Waterboarding was used for interrogation at least as early as the Spanish Inquisition to obtain information, coerce confessions, punish, and intimidate. It is considered to be torture by a wide range of authorities, including legal experts, politicians, war veterans, According to Republican United States Senator and 2008 presidential candidate John McCain, who was tortured as a prisoner of war in North Vietnam, waterboarding is "torture, no different than holding a pistol to his head and firing a blank" and can damage the subject's psyche "in ways that may never heal." intelligence officials, military judges, "Waterboarding is inhumane, it is torture, and it is illegal." and "Waterboarding detainees amounts to illegal torture in all circumstances.". From Rear Admiral Donald J. Guter, United States Navy (Ret.) Judge Advocate General of the Navy, 2000-02; Rear Admiral John D. Hutson, United States Navy (Ret.) Judge Advocate General of the Navy, 1997-2000; Major General John L. Fugh, United States Army (Ret.) Judge Advocate General of the Army, 1991-93; Brigadier General David M. Brahms, United States Marine Corps (Ret.) Staff Judge Advocate to the Commandant, 1985-88. and human rights organizations. "There is no doubt that waterboarding is torture, despite the administration’s reluctance to say so,” Kenneth Roth, executive director of Human Rights Watch. In 2007 waterboarding led to a political scandal in the United States when the press reported that the CIA had waterboarded extrajudicial prisoners and that the Justice Department had authorized this procedure. The CIA has admitted waterboarding Al-Qaida suspects Khalid Sheikh Mohammed, Abu Zubaydah and Abd al-Rahim al-Nashiri.