John Brennan, 57, currently serves as U.S. President Barack Obama's chief adviser on counterterrorism.
He joined Obama's National Security Council after 25 years in the intelligence community and a career which saw him rise steadily up the ranks of the Central Intelligence Agency (CIA).
Among other roles, he served as chief of staff to former CIA Director George Tenet and was CIA station chief in Saudi Arabia before becoming deputy executive director of the agency during President George W. Bush’s first term.
He was the first director of the National Counterterrorism Center, in 2004-05.
Brennan has played a key role in Washington's efforts to disrupt Al-Qaeda since the attacks on September 11, 2001, including advising Obama on his decision to launch a raid on Osama bin Laden's Pakistani compound in May 2011.
According to "The Washington Post," Brennan is "known to have the full trust and confidence of the president" and has "been involved in virtually all major national security issues" during the past four years.
In announcing Brennan's nomination, Obama called him the hardest working public servant he has ever known.
Drone Strike Program
Within the Obama administration, Brennan has led the transformation of the U.S. approach to counterterrorism from that of a ground war in Afghanistan and more traditional methods of offense to targeted killings of high-priority terrorist operatives, often involving the use of drones, in countries such as Pakistan, Yemen, and Somalia.
Brennan was the first Obama administration official to acknowledge publicly that the drone strike program existed.
"Unfortunately, sometimes you have to take life to save lives, and that's what we've been able to do to prevent these individual terrorists from carrying out their murderous attacks," Brennan said on the U.S. television network ABC in 2012.
At the same time, and amid resistance from the intelligence community, he has advocated that the military retain primary responsibility for the drone program, with the CIA focusing solely on the intelligence side.
Brennan was considered Obama's top candidate to lead the CIA in 2009, but he withdrew his name from consideration after human rights activists accused him of supporting or condoning the use of waterboarding and other Bush-era enhanced interrogation techniques. He has called that criticism unfair.
Brennan has since spoken out for the need for oversight of U.S. counterterrorism methods and has argued in favor of closing the U.S. prison at Guantanamo Bay.
Brennan is a native of the northeastern state of New Jersey. He has a bachelor's degree in political science from Fordham University and a master's degree in government from the University of Texas at Austin. He is fluent in Arabic. He and his wife, Katy Pokluda Brennan, have three children.
-- Richard Solash with reporting by "The New York Times," "The Washington Post," and "The Huffington Post"